[PATCH RFC 0/4] kvm/mm: Allow GUP to respond to non fatal signals

[PATCH RFC 0/4] kvm/mm: Allow GUP to respond to non fatal signals

[Peter Xu]

[Marked as RFC for now]

One issue was reported that libvirt won't be able to stop the virtual
machine using QMP command "stop" during a paused postcopy migration [1].

It won't work because "stop the VM" operation requires the hypervisor to
kick all the vcpu threads out using SIG_IPI in QEMU (which is translated to
a SIGUSR1).  However since during a paused postcopy, the vcpu threads are
hang death at handle_userfault() so there're simply not responding to the
kicks.  Further, the "stop" command will further hang the QMP channel.

The mm has facility to process generic signal (FAULT_FLAG_INTERRUPTIBLE),
however it's only used in the PF handlers only, not in GUP. Unluckily, KVM
is a heavy GUP user on guest page faults.  It means we won't be able to
interrupt a long page fault for KVM fetching guest pages with what we have
right now.

I think it's reasonable for GUP to only listen to fatal signals, as most of
the GUP users are not really ready to handle such case.  But actually KVM
is not such an user, and KVM actually has rich infrastructure to handle
even generic signals, and properly deliver the signal to the userspace.
Then the page fault can be retried in the next KVM_RUN.

This patchset added FOLL_INTERRUPTIBLE to enable FAULT_FLAG_INTERRUPTIBLE,
and let KVM be the first one to use it.

Tests
=====

I created a postcopy environment, pause the migration by shutting down the
network to emulate a network failure (so the handle_userfault() will stuck
for a long time), then I tried three things:

  (1) Sending QMP command "stop" to QEMU monitor,
  (2) Hitting Ctrl-C from QEMU cmdline,
  (3) GDB attach to the dest QEMU process.

Before this patchset, all three use case hang.  After the patchset, all
work just like when there's not network failure at all.

Please have a look, thanks.

[1] https://gitlab.com/qemu-project/qemu/-/issues/1052

Peter Xu (4):
  mm/gup: Add FOLL_INTERRUPTIBLE
  kvm: Merge "atomic" and "write" in __gfn_to_pfn_memslot()
  kvm: Add new pfn error KVM_PFN_ERR_INTR
  kvm/x86: Allow to respond to generic signals during slow page faults

 arch/arm64/kvm/mmu.c                   |  5 ++--
 arch/powerpc/kvm/book3s_64_mmu_hv.c    |  5 ++--
 arch/powerpc/kvm/book3s_64_mmu_radix.c |  5 ++--
 arch/x86/kvm/mmu/mmu.c                 | 19 ++++++++----
 include/linux/kvm_host.h               | 21 ++++++++++++-
 include/linux/mm.h                     |  1 +
 mm/gup.c                               | 33 ++++++++++++++++++---
 virt/kvm/kvm_main.c                    | 41 ++++++++++++++++----------
 virt/kvm/kvm_mm.h                      |  6 ++--
 virt/kvm/pfncache.c                    |  2 +-
 10 files changed, 104 insertions(+), 34 deletions(-)

-- 
2.32.0

[PATCH RFC 1/4] mm/gup: Add FOLL_INTERRUPTIBLE

[Peter Xu]

We have had FAULT_FLAG_INTERRUPTIBLE but it was never applied to GUPs.  One
issue with it is that not all GUP paths are able to handle signal delivers
besides SIGKILL.

That's not ideal for the GUP users who are actually able to handle these
cases, like KVM.

KVM uses GUP extensively on faulting guest pages, during which we've got
existing infrastructures to retry a page fault at a later time.  Allowing
the GUP to be interrupted by generic signals can make KVM related threads
to be more responsive.  For examples:

  (1) SIGUSR1: which QEMU/KVM uses to deliver an inter-process IPI,
      e.g. when the admin issues a vm_stop QMP command, SIGUSR1 can be
      generated to kick the vcpus out of kernel context immediately,

  (2) SIGINT: which can be used with interactive hypervisor users to stop a
      virtual machine with Ctrl-C without any delays/hangs,

  (3) SIGTRAP: which grants GDB capability even during page faults that are
      stuck for a long time.

Normally hypervisor will be able to receive these signals properly, but not
if we're stuck in a GUP for a long time for whatever reason.  It happens
easily with a stucked postcopy migration when e.g. a network temp failure
happens, then some vcpu threads can hang death waiting for the pages.  With
the new FOLL_INTERRUPTIBLE, we can allow GUP users like KVM to selectively
enable the ability to trap these signals.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/linux/mm.h |  1 +
 mm/gup.c           | 33 +++++++++++++++++++++++++++++----
 2 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index bc8f326be0ce..ebdf8a6b86c1 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2941,6 +2941,7 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
 #define FOLL_SPLIT_PMD	0x20000	/* split huge pmd before returning */
 #define FOLL_PIN	0x40000	/* pages must be released via unpin_user_page */
 #define FOLL_FAST_ONLY	0x80000	/* gup_fast: prevent fall-back to slow gup */
+#define FOLL_INTERRUPTIBLE  0x100000 /* allow interrupts from generic signals */
 
 /*
  * FOLL_PIN and FOLL_LONGTERM may be used in various combinations with each
diff --git a/mm/gup.c b/mm/gup.c
index 551264407624..ad74b137d363 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -933,8 +933,17 @@ static int faultin_page(struct vm_area_struct *vma,
 		fault_flags |= FAULT_FLAG_WRITE;
 	if (*flags & FOLL_REMOTE)
 		fault_flags |= FAULT_FLAG_REMOTE;
-	if (locked)
+	if (locked) {
 		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+		/*
+		 * We should only grant FAULT_FLAG_INTERRUPTIBLE when we're
+		 * (at least) killable.  It also mostly means we're not
+		 * with NOWAIT.  Otherwise ignore FOLL_INTERRUPTIBLE since
+		 * it won't make a lot of sense to be used alone.
+		 */
+		if (*flags & FOLL_INTERRUPTIBLE)
+			fault_flags |= FAULT_FLAG_INTERRUPTIBLE;
+	}
 	if (*flags & FOLL_NOWAIT)
 		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
 	if (*flags & FOLL_TRIED) {
@@ -1322,6 +1331,22 @@ int fixup_user_fault(struct mm_struct *mm,
 }
 EXPORT_SYMBOL_GPL(fixup_user_fault);
 
+/*
+ * GUP always responds to fatal signals.  When FOLL_INTERRUPTIBLE is
+ * specified, it'll also respond to generic signals.  The caller of GUP
+ * that has FOLL_INTERRUPTIBLE should take care of the GUP interruption.
+ */
+static bool gup_signal_pending(unsigned int flags)
+{
+	if (fatal_signal_pending(current))
+		return true;
+
+	if (!(flags & FOLL_INTERRUPTIBLE))
+		return false;
+
+	return signal_pending(current);
+}
+
 /*
  * Please note that this function, unlike __get_user_pages will not
  * return 0 for nr_pages > 0 without FOLL_NOWAIT
@@ -1403,11 +1428,11 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
 		 * Repeat on the address that fired VM_FAULT_RETRY
 		 * with both FAULT_FLAG_ALLOW_RETRY and
 		 * FAULT_FLAG_TRIED.  Note that GUP can be interrupted
-		 * by fatal signals, so we need to check it before we
+		 * by fatal signals of even common signals, depending on
+		 * the caller's request. So we need to check it before we
 		 * start trying again otherwise it can loop forever.
 		 */
-
-		if (fatal_signal_pending(current)) {
+		if (gup_signal_pending(flags)) {
 			if (!pages_done)
 				pages_done = -EINTR;
 			break;
-- 
2.32.0

[PATCH RFC 2/4] kvm: Merge "atomic" and "write" in __gfn_to_pfn_memslot()

[Peter Xu]

Merge two boolean parameters into a bitmask flag called kvm_gtp_flag_t for
__gfn_to_pfn_memslot().  This cleans the parameter lists, and also prepare
for new boolean to be added to __gfn_to_pfn_memslot().

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 arch/arm64/kvm/mmu.c                   |  5 ++--
 arch/powerpc/kvm/book3s_64_mmu_hv.c    |  5 ++--
 arch/powerpc/kvm/book3s_64_mmu_radix.c |  5 ++--
 arch/x86/kvm/mmu/mmu.c                 | 10 +++----
 include/linux/kvm_host.h               |  9 ++++++-
 virt/kvm/kvm_main.c                    | 37 +++++++++++++++-----------
 virt/kvm/kvm_mm.h                      |  6 +++--
 virt/kvm/pfncache.c                    |  2 +-
 8 files changed, 49 insertions(+), 30 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index f5651a05b6a8..ce8066ded15b 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1204,8 +1204,9 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	 */
 	smp_rmb();
 
-	pfn = __gfn_to_pfn_memslot(memslot, gfn, false, NULL,
-				   write_fault, &writable, NULL);
+	pfn = __gfn_to_pfn_memslot(memslot, gfn,
+				   write_fault ? KVM_GTP_WRITE : 0,
+				   false, NULL, &writable, NULL);
 	if (pfn == KVM_PFN_ERR_HWPOISON) {
 		kvm_send_hwpoison_signal(hva, vma_shift);
 		return 0;
diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
index 514fd45c1994..2f5fad2e1b7f 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
@@ -598,8 +598,9 @@ int kvmppc_book3s_hv_page_fault(struct kvm_vcpu *vcpu,
 		write_ok = true;
 	} else {
 		/* Call KVM generic code to do the slow-path check */
-		pfn = __gfn_to_pfn_memslot(memslot, gfn, false, NULL,
-					   writing, &write_ok, NULL);
+		pfn = __gfn_to_pfn_memslot(memslot, gfn,
+					   writing ? KVM_GTP_WRITE : 0,
+					   false, NULL, &write_ok, NULL);
 		if (is_error_noslot_pfn(pfn))
 			return -EFAULT;
 		page = NULL;
diff --git a/arch/powerpc/kvm/book3s_64_mmu_radix.c b/arch/powerpc/kvm/book3s_64_mmu_radix.c
index 42851c32ff3b..232b17c75b83 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_radix.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_radix.c
@@ -845,8 +845,9 @@ int kvmppc_book3s_instantiate_page(struct kvm_vcpu *vcpu,
 		unsigned long pfn;
 
 		/* Call KVM generic code to do the slow-path check */
-		pfn = __gfn_to_pfn_memslot(memslot, gfn, false, NULL,
-					   writing, upgrade_p, NULL);
+		pfn = __gfn_to_pfn_memslot(memslot, gfn,
+					   writing ? KVM_GTP_WRITE : 0,
+					   NULL, upgrade_p, NULL);
 		if (is_error_noslot_pfn(pfn))
 			return -EFAULT;
 		page = NULL;
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index f4653688fa6d..e92f1ab63d6a 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3968,6 +3968,7 @@ void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu, struct kvm_async_pf *work)
 
 static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 {
+	kvm_gtp_flag_t flags = fault->write ? KVM_GTP_WRITE : 0;
 	struct kvm_memory_slot *slot = fault->slot;
 	bool async;
 
@@ -3999,8 +4000,8 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 	}
 
 	async = false;
-	fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, &async,
-					  fault->write, &fault->map_writable,
+	fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, flags,
+					  &async, &fault->map_writable,
 					  &fault->hva);
 	if (!async)
 		return RET_PF_CONTINUE; /* *pfn has correct page already */
@@ -4016,9 +4017,8 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 		}
 	}
 
-	fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, NULL,
-					  fault->write, &fault->map_writable,
-					  &fault->hva);
+	fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, flags, NULL,
+					  &fault->map_writable, &fault->hva);
 	return RET_PF_CONTINUE;
 }
 
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index c20f2d55840c..b646b6fcaec6 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1146,8 +1146,15 @@ kvm_pfn_t gfn_to_pfn_prot(struct kvm *kvm, gfn_t gfn, bool write_fault,
 		      bool *writable);
 kvm_pfn_t gfn_to_pfn_memslot(const struct kvm_memory_slot *slot, gfn_t gfn);
 kvm_pfn_t gfn_to_pfn_memslot_atomic(const struct kvm_memory_slot *slot, gfn_t gfn);
+
+/* gfn_to_pfn (gtp) flags */
+typedef unsigned int __bitwise kvm_gtp_flag_t;
+
+#define  KVM_GTP_WRITE          ((__force kvm_gtp_flag_t) BIT(0))
+#define  KVM_GTP_ATOMIC         ((__force kvm_gtp_flag_t) BIT(1))
+
 kvm_pfn_t __gfn_to_pfn_memslot(const struct kvm_memory_slot *slot, gfn_t gfn,
-			       bool atomic, bool *async, bool write_fault,
+			       kvm_gtp_flag_t gtp_flags, bool *async,
 			       bool *writable, hva_t *hva);
 
 void kvm_release_pfn_clean(kvm_pfn_t pfn);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 64ec2222a196..952400b42ee9 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2444,9 +2444,11 @@ static bool hva_to_pfn_fast(unsigned long addr, bool write_fault,
  * The slow path to get the pfn of the specified host virtual address,
  * 1 indicates success, -errno is returned if error is detected.
  */
-static int hva_to_pfn_slow(unsigned long addr, bool *async, bool write_fault,
+static int hva_to_pfn_slow(unsigned long addr, bool *async,
+			   kvm_gtp_flag_t gtp_flags,
 			   bool *writable, kvm_pfn_t *pfn)
 {
+	bool write_fault = gtp_flags & KVM_GTP_WRITE;
 	unsigned int flags = FOLL_HWPOISON;
 	struct page *page;
 	int npages = 0;
@@ -2565,20 +2567,22 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
 /*
  * Pin guest page in memory and return its pfn.
  * @addr: host virtual address which maps memory to the guest
- * @atomic: whether this function can sleep
+ * @gtp_flags: kvm_gtp_flag_t flags (atomic, write, ..)
  * @async: whether this function need to wait IO complete if the
  *         host page is not in the memory
- * @write_fault: whether we should get a writable host page
  * @writable: whether it allows to map a writable host page for !@write_fault
  *
- * The function will map a writable host page for these two cases:
+ * The function will map a writable (KVM_GTP_WRITE set) host page for these
+ * two cases:
  * 1): @write_fault = true
  * 2): @write_fault = false && @writable, @writable will tell the caller
  *     whether the mapping is writable.
  */
-kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
-		     bool write_fault, bool *writable)
+kvm_pfn_t hva_to_pfn(unsigned long addr, kvm_gtp_flag_t gtp_flags, bool *async,
+		     bool *writable)
 {
+	bool write_fault = gtp_flags & KVM_GTP_WRITE;
+	bool atomic = gtp_flags & KVM_GTP_ATOMIC;
 	struct vm_area_struct *vma;
 	kvm_pfn_t pfn = 0;
 	int npages, r;
@@ -2592,7 +2596,7 @@ kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
 	if (atomic)
 		return KVM_PFN_ERR_FAULT;
 
-	npages = hva_to_pfn_slow(addr, async, write_fault, writable, &pfn);
+	npages = hva_to_pfn_slow(addr, async, gtp_flags, writable, &pfn);
 	if (npages == 1)
 		return pfn;
 
@@ -2625,10 +2629,11 @@ kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
 }
 
 kvm_pfn_t __gfn_to_pfn_memslot(const struct kvm_memory_slot *slot, gfn_t gfn,
-			       bool atomic, bool *async, bool write_fault,
+			       kvm_gtp_flag_t gtp_flags, bool *async,
 			       bool *writable, hva_t *hva)
 {
-	unsigned long addr = __gfn_to_hva_many(slot, gfn, NULL, write_fault);
+	unsigned long addr = __gfn_to_hva_many(slot, gfn, NULL,
+					       gtp_flags & KVM_GTP_WRITE);
 
 	if (hva)
 		*hva = addr;
@@ -2651,28 +2656,30 @@ kvm_pfn_t __gfn_to_pfn_memslot(const struct kvm_memory_slot *slot, gfn_t gfn,
 		writable = NULL;
 	}
 
-	return hva_to_pfn(addr, atomic, async, write_fault,
-			  writable);
+	return hva_to_pfn(addr, gtp_flags, async, writable);
 }
 EXPORT_SYMBOL_GPL(__gfn_to_pfn_memslot);
 
 kvm_pfn_t gfn_to_pfn_prot(struct kvm *kvm, gfn_t gfn, bool write_fault,
 		      bool *writable)
 {
-	return __gfn_to_pfn_memslot(gfn_to_memslot(kvm, gfn), gfn, false, NULL,
-				    write_fault, writable, NULL);
+	return __gfn_to_pfn_memslot(gfn_to_memslot(kvm, gfn), gfn,
+				    write_fault ? KVM_GTP_WRITE : 0,
+				    NULL, writable, NULL);
 }
 EXPORT_SYMBOL_GPL(gfn_to_pfn_prot);
 
 kvm_pfn_t gfn_to_pfn_memslot(const struct kvm_memory_slot *slot, gfn_t gfn)
 {
-	return __gfn_to_pfn_memslot(slot, gfn, false, NULL, true, NULL, NULL);
+	return __gfn_to_pfn_memslot(slot, gfn, KVM_GTP_WRITE,
+				    NULL, NULL, NULL);
 }
 EXPORT_SYMBOL_GPL(gfn_to_pfn_memslot);
 
 kvm_pfn_t gfn_to_pfn_memslot_atomic(const struct kvm_memory_slot *slot, gfn_t gfn)
 {
-	return __gfn_to_pfn_memslot(slot, gfn, true, NULL, true, NULL, NULL);
+	return __gfn_to_pfn_memslot(slot, gfn, KVM_GTP_WRITE | KVM_GTP_ATOMIC,
+				    NULL, NULL, NULL);
 }
 EXPORT_SYMBOL_GPL(gfn_to_pfn_memslot_atomic);
 
diff --git a/virt/kvm/kvm_mm.h b/virt/kvm/kvm_mm.h
index 41da467d99c9..1c870911eb48 100644
--- a/virt/kvm/kvm_mm.h
+++ b/virt/kvm/kvm_mm.h
@@ -3,6 +3,8 @@
 #ifndef __KVM_MM_H__
 #define __KVM_MM_H__ 1
 
+#include <linux/kvm_host.h>
+
 /*
  * Architectures can choose whether to use an rwlock or spinlock
  * for the mmu_lock.  These macros, for use in common code
@@ -24,8 +26,8 @@
 #define KVM_MMU_READ_UNLOCK(kvm)	spin_unlock(&(kvm)->mmu_lock)
 #endif /* KVM_HAVE_MMU_RWLOCK */
 
-kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
-		     bool write_fault, bool *writable);
+kvm_pfn_t hva_to_pfn(unsigned long addr, kvm_gtp_flag_t gtp_flags, bool *async,
+		     bool *writable);
 
 #ifdef CONFIG_HAVE_KVM_PFNCACHE
 void gfn_to_pfn_cache_invalidate_start(struct kvm *kvm,
diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index dd84676615f1..0f9f6b5d2fbb 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -123,7 +123,7 @@ static kvm_pfn_t hva_to_pfn_retry(struct kvm *kvm, unsigned long uhva)
 		smp_rmb();
 
 		/* We always request a writeable mapping */
-		new_pfn = hva_to_pfn(uhva, false, NULL, true, NULL);
+		new_pfn = hva_to_pfn(uhva, KVM_GTP_WRITE, NULL, NULL);
 		if (is_error_noslot_pfn(new_pfn))
 			break;
 
-- 
2.32.0

[PATCH RFC 3/4] kvm: Add new pfn error KVM_PFN_ERR_INTR

[Peter Xu]

Add one new PFN error type to show when we cannot finish fetching the PFN
due to interruptions.  For example, by receiving a generic signal.

This prepares KVM to be able to respond to SIGUSR1 (for QEMU that's the
SIGIPI) even during e.g. handling an userfaultfd page fault.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/linux/kvm_host.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index b646b6fcaec6..4f84a442f67f 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -96,6 +96,7 @@
 #define KVM_PFN_ERR_FAULT	(KVM_PFN_ERR_MASK)
 #define KVM_PFN_ERR_HWPOISON	(KVM_PFN_ERR_MASK + 1)
 #define KVM_PFN_ERR_RO_FAULT	(KVM_PFN_ERR_MASK + 2)
+#define KVM_PFN_ERR_INTR	(KVM_PFN_ERR_MASK + 3)
 
 /*
  * error pfns indicate that the gfn is in slot but faild to
@@ -106,6 +107,16 @@ static inline bool is_error_pfn(kvm_pfn_t pfn)
 	return !!(pfn & KVM_PFN_ERR_MASK);
 }
 
+/*
+ * When KVM_PFN_ERR_INTR is returned, it means we're interrupted during
+ * fetching the PFN (e.g. a signal might have arrived), so we may want to
+ * retry at some later point and kick the userspace to handle the signal.
+ */
+static inline bool is_intr_pfn(kvm_pfn_t pfn)
+{
+	return pfn == KVM_PFN_ERR_INTR;
+}
+
 /*
  * error_noslot pfns indicate that the gfn can not be
  * translated to pfn - it is not in slot or failed to
-- 
2.32.0

[PATCH RFC 4/4] kvm/x86: Allow to respond to generic signals during slow page faults

[Peter Xu]

All the facilities should be ready for this, what we need to do is to add a
new KVM_GTP_INTERRUPTIBLE flag showing that we're willing to be interrupted
by common signals during the __gfn_to_pfn_memslot() request, and wire it up
with a FOLL_INTERRUPTIBLE flag that we've just introduced.

Note that only x86 slow page fault routine will set this new bit.  The new
bit is not used in non-x86 arch or on other gup paths even for x86.
However it can actually be used elsewhere too but not yet covered.

When we see the PFN fetching was interrupted, do early exit to userspace
with an KVM_EXIT_INTR exit reason.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 arch/x86/kvm/mmu/mmu.c   | 9 +++++++++
 include/linux/kvm_host.h | 1 +
 virt/kvm/kvm_main.c      | 4 ++++
 3 files changed, 14 insertions(+)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index e92f1ab63d6a..b39acb7cb16d 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3012,6 +3012,13 @@ static int kvm_handle_bad_page(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn)
 static int handle_abnormal_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
 			       unsigned int access)
 {
+	/* NOTE: not all error pfn is fatal; handle intr before the other ones */
+	if (unlikely(is_intr_pfn(fault->pfn))) {
+		vcpu->run->exit_reason = KVM_EXIT_INTR;
+		++vcpu->stat.signal_exits;
+		return -EINTR;
+	}
+
 	/* The pfn is invalid, report the error! */
 	if (unlikely(is_error_pfn(fault->pfn)))
 		return kvm_handle_bad_page(vcpu, fault->gfn, fault->pfn);
@@ -4017,6 +4024,8 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 		}
 	}
 
+	/* Allow to respond to generic signals in slow page faults */
+	flags |= KVM_GTP_INTERRUPTIBLE;
 	fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, flags, NULL,
 					  &fault->map_writable, &fault->hva);
 	return RET_PF_CONTINUE;
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 4f84a442f67f..c8d98e435537 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1163,6 +1163,7 @@ typedef unsigned int __bitwise kvm_gtp_flag_t;
 
 #define  KVM_GTP_WRITE          ((__force kvm_gtp_flag_t) BIT(0))
 #define  KVM_GTP_ATOMIC         ((__force kvm_gtp_flag_t) BIT(1))
+#define  KVM_GTP_INTERRUPTIBLE  ((__force kvm_gtp_flag_t) BIT(2))
 
 kvm_pfn_t __gfn_to_pfn_memslot(const struct kvm_memory_slot *slot, gfn_t gfn,
 			       kvm_gtp_flag_t gtp_flags, bool *async,
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 952400b42ee9..b3873cac5672 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2462,6 +2462,8 @@ static int hva_to_pfn_slow(unsigned long addr, bool *async,
 		flags |= FOLL_WRITE;
 	if (async)
 		flags |= FOLL_NOWAIT;
+	if (gtp_flags & KVM_GTP_INTERRUPTIBLE)
+		flags |= FOLL_INTERRUPTIBLE;
 
 	npages = get_user_pages_unlocked(addr, 1, &page, flags);
 	if (npages != 1)
@@ -2599,6 +2601,8 @@ kvm_pfn_t hva_to_pfn(unsigned long addr, kvm_gtp_flag_t gtp_flags, bool *async,
 	npages = hva_to_pfn_slow(addr, async, gtp_flags, writable, &pfn);
 	if (npages == 1)
 		return pfn;
+	if (npages == -EINTR)
+		return KVM_PFN_ERR_INTR;
 
 	mmap_read_lock(current->mm);
 	if (npages == -EHWPOISON ||
-- 
2.32.0

Re: [PATCH RFC 1/4] mm/gup: Add FOLL_INTERRUPTIBLE

[David Hildenbrand]

On 17.06.22 03:41, Peter Xu wrote:
> We have had FAULT_FLAG_INTERRUPTIBLE but it was never applied to GUPs.  One
> issue with it is that not all GUP paths are able to handle signal delivers
> besides SIGKILL.
> 
> That's not ideal for the GUP users who are actually able to handle these
> cases, like KVM.
> 
> KVM uses GUP extensively on faulting guest pages, during which we've got
> existing infrastructures to retry a page fault at a later time.  Allowing
> the GUP to be interrupted by generic signals can make KVM related threads
> to be more responsive.  For examples:
> 
>   (1) SIGUSR1: which QEMU/KVM uses to deliver an inter-process IPI,
>       e.g. when the admin issues a vm_stop QMP command, SIGUSR1 can be
>       generated to kick the vcpus out of kernel context immediately,
> 
>   (2) SIGINT: which can be used with interactive hypervisor users to stop a
>       virtual machine with Ctrl-C without any delays/hangs,
> 
>   (3) SIGTRAP: which grants GDB capability even during page faults that are
>       stuck for a long time.
> 
> Normally hypervisor will be able to receive these signals properly, but not
> if we're stuck in a GUP for a long time for whatever reason.  It happens
> easily with a stucked postcopy migration when e.g. a network temp failure
> happens, then some vcpu threads can hang death waiting for the pages.  With
> the new FOLL_INTERRUPTIBLE, we can allow GUP users like KVM to selectively
> enable the ability to trap these signals.

This makes sense to me. I assume relevant callers will detect "GUP
failed" but also "well, there is a signal to handle" and cleanly back
off, correct?

-- 
Thanks,

David / dhildenb

Re: [PATCH RFC 1/4] mm/gup: Add FOLL_INTERRUPTIBLE

[Peter Xu]

On Tue, Jun 21, 2022 at 10:23:32AM +0200, David Hildenbrand wrote:
> On 17.06.22 03:41, Peter Xu wrote:
> > We have had FAULT_FLAG_INTERRUPTIBLE but it was never applied to GUPs.  One
> > issue with it is that not all GUP paths are able to handle signal delivers
> > besides SIGKILL.
> > 
> > That's not ideal for the GUP users who are actually able to handle these
> > cases, like KVM.
> > 
> > KVM uses GUP extensively on faulting guest pages, during which we've got
> > existing infrastructures to retry a page fault at a later time.  Allowing
> > the GUP to be interrupted by generic signals can make KVM related threads
> > to be more responsive.  For examples:
> > 
> >   (1) SIGUSR1: which QEMU/KVM uses to deliver an inter-process IPI,
> >       e.g. when the admin issues a vm_stop QMP command, SIGUSR1 can be
> >       generated to kick the vcpus out of kernel context immediately,
> > 
> >   (2) SIGINT: which can be used with interactive hypervisor users to stop a
> >       virtual machine with Ctrl-C without any delays/hangs,
> > 
> >   (3) SIGTRAP: which grants GDB capability even during page faults that are
> >       stuck for a long time.
> > 
> > Normally hypervisor will be able to receive these signals properly, but not
> > if we're stuck in a GUP for a long time for whatever reason.  It happens
> > easily with a stucked postcopy migration when e.g. a network temp failure
> > happens, then some vcpu threads can hang death waiting for the pages.  With
> > the new FOLL_INTERRUPTIBLE, we can allow GUP users like KVM to selectively
> > enable the ability to trap these signals.
> 
> This makes sense to me. I assume relevant callers will detect "GUP
> failed" but also "well, there is a signal to handle" and cleanly back
> off, correct?

Correct, via an -EINTR.

One thing to mention is that the gup user behavior will be the same as
before if the caller didn't explicilty pass in FOLL_INTERRUPTIBLE with the
gup call.  So after the whole series applied only kvm (and only some path
of kvm, not all GUP; I only touched up the x86 slow page fault path) to
handle this, but that'll be far enough to cover 99.99% use cases that I
wanted to take care of.

E.g., some kvm request to gup on some guest apic page may not still be able
to respond to a SIGUSR1 but that's very very rare, and we can always add
more users of FOLL_INTERRUPTIBLE when the code is ready to benefit from the
fast respondings.

Thanks,

-- 
Peter Xu