[PATCH v2 0/3] xen: remove some checks for always present Xen features

[PATCH v2 0/3] xen: remove some checks for always present Xen features

[Juergen Gross]

Some features of Xen can be assumed to be always present, so add a
central check to verify this being true and remove the other checks.

Juergen Gross (3):
  xen: check required Xen features
  xen: assume XENFEAT_mmu_pt_update_preserve_ad being set for pv guests
  xen: assume XENFEAT_gnttab_map_avail_bits being set for pv guests

 arch/x86/xen/enlighten_pv.c | 12 ++----------
 arch/x86/xen/mmu_pv.c       |  4 ++--
 drivers/xen/features.c      | 18 ++++++++++++++++++
 drivers/xen/gntdev.c        | 36 ++----------------------------------
 4 files changed, 24 insertions(+), 46 deletions(-)

-- 
2.26.2

[PATCH v2 1/3] xen: check required Xen features

[Juergen Gross]

Linux kernel is not supported to run on Xen versions older than 4.0.

Add tests for required Xen features always being present in Xen 4.0
and newer.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
V2:
- rename macro (Boris Ostrovsky)
- panic() in case of missing feature (Boris Ostrovsky)
---
 drivers/xen/features.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/drivers/xen/features.c b/drivers/xen/features.c
index 25c053b09605..7b591443833c 100644
--- a/drivers/xen/features.c
+++ b/drivers/xen/features.c
@@ -9,13 +9,26 @@
 #include <linux/types.h>
 #include <linux/cache.h>
 #include <linux/export.h>
+#include <linux/printk.h>
 
 #include <asm/xen/hypercall.h>
 
+#include <xen/xen.h>
 #include <xen/interface/xen.h>
 #include <xen/interface/version.h>
 #include <xen/features.h>
 
+/*
+ * Linux kernel expects at least Xen 4.0.
+ *
+ * Assume some features to be available for that reason (depending on guest
+ * mode, of course).
+ */
+#define chk_required_feature(f) {					\
+		if (!xen_feature(f))					\
+			panic("Xen: feature %s not available!\n", #f);	\
+	}
+
 u8 xen_features[XENFEAT_NR_SUBMAPS * 32] __read_mostly;
 EXPORT_SYMBOL_GPL(xen_features);
 
@@ -31,4 +44,9 @@ void xen_setup_features(void)
 		for (j = 0; j < 32; j++)
 			xen_features[i * 32 + j] = !!(fi.submap & 1<<j);
 	}
+
+	if (xen_pv_domain()) {
+		chk_required_feature(XENFEAT_mmu_pt_update_preserve_ad);
+		chk_required_feature(XENFEAT_gnttab_map_avail_bits);
+	}
 }
-- 
2.26.2

[PATCH v2 2/3] xen: assume XENFEAT_mmu_pt_update_preserve_ad being set for pv guests

[Juergen Gross]

XENFEAT_mmu_pt_update_preserve_ad is always set in Xen 4.0 and newer.
Remove coding assuming it might be zero.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/xen/enlighten_pv.c | 12 ++----------
 arch/x86/xen/mmu_pv.c       |  4 ++--
 2 files changed, 4 insertions(+), 12 deletions(-)

diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index 03149422dce2..753f63734c13 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -116,9 +116,8 @@ static void __init xen_banner(void)
 	HYPERVISOR_xen_version(XENVER_extraversion, &extra);
 
 	pr_info("Booting paravirtualized kernel on %s\n", pv_info.name);
-	printk(KERN_INFO "Xen version: %d.%d%s%s\n",
-	       version >> 16, version & 0xffff, extra.extraversion,
-	       xen_feature(XENFEAT_mmu_pt_update_preserve_ad) ? " (preserve-AD)" : "");
+	pr_info("Xen version: %d.%d%s (preserve-AD)\n",
+		version >> 16, version & 0xffff, extra.extraversion);
 }
 
 static void __init xen_pv_init_platform(void)
@@ -1302,13 +1301,6 @@ asmlinkage __visible void __init xen_start_kernel(void)
 	xen_init_apic();
 #endif
 
-	if (xen_feature(XENFEAT_mmu_pt_update_preserve_ad)) {
-		pv_ops.mmu.ptep_modify_prot_start =
-			xen_ptep_modify_prot_start;
-		pv_ops.mmu.ptep_modify_prot_commit =
-			xen_ptep_modify_prot_commit;
-	}
-
 	machine_ops = xen_machine_ops;
 
 	/*
diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
index ade789e73ee4..1df5f01529e5 100644
--- a/arch/x86/xen/mmu_pv.c
+++ b/arch/x86/xen/mmu_pv.c
@@ -2099,8 +2099,8 @@ static const struct pv_mmu_ops xen_mmu_ops __initconst = {
 	.set_pte = xen_set_pte_init,
 	.set_pmd = xen_set_pmd_hyper,
 
-	.ptep_modify_prot_start = __ptep_modify_prot_start,
-	.ptep_modify_prot_commit = __ptep_modify_prot_commit,
+	.ptep_modify_prot_start = xen_ptep_modify_prot_start,
+	.ptep_modify_prot_commit = xen_ptep_modify_prot_commit,
 
 	.pte_val = PV_CALLEE_SAVE(xen_pte_val),
 	.pgd_val = PV_CALLEE_SAVE(xen_pgd_val),
-- 
2.26.2

[PATCH v2 3/3] xen: assume XENFEAT_gnttab_map_avail_bits being set for pv guests

[Juergen Gross]

XENFEAT_gnttab_map_avail_bits is always set in Xen 4.0 and newer.
Remove coding assuming it might be zero.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 drivers/xen/gntdev.c | 36 ++----------------------------------
 1 file changed, 2 insertions(+), 34 deletions(-)

diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index a3e7be96527d..1e7f6b1c0c97 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -266,20 +266,13 @@ static int find_grant_ptes(pte_t *pte, unsigned long addr, void *data)
 {
 	struct gntdev_grant_map *map = data;
 	unsigned int pgnr = (addr - map->vma->vm_start) >> PAGE_SHIFT;
-	int flags = map->flags | GNTMAP_application_map | GNTMAP_contains_pte;
+	int flags = map->flags | GNTMAP_application_map | GNTMAP_contains_pte |
+		    (1 << _GNTMAP_guest_avail0);
 	u64 pte_maddr;
 
 	BUG_ON(pgnr >= map->count);
 	pte_maddr = arbitrary_virt_to_machine(pte).maddr;
 
-	/*
-	 * Set the PTE as special to force get_user_pages_fast() fall
-	 * back to the slow path.  If this is not supported as part of
-	 * the grant map, it will be done afterwards.
-	 */
-	if (xen_feature(XENFEAT_gnttab_map_avail_bits))
-		flags |= (1 << _GNTMAP_guest_avail0);
-
 	gnttab_set_map_op(&map->map_ops[pgnr], pte_maddr, flags,
 			  map->grants[pgnr].ref,
 			  map->grants[pgnr].domid);
@@ -288,14 +281,6 @@ static int find_grant_ptes(pte_t *pte, unsigned long addr, void *data)
 	return 0;
 }
 
-#ifdef CONFIG_X86
-static int set_grant_ptes_as_special(pte_t *pte, unsigned long addr, void *data)
-{
-	set_pte_at(current->mm, addr, pte, pte_mkspecial(*pte));
-	return 0;
-}
-#endif
-
 int gntdev_map_grant_pages(struct gntdev_grant_map *map)
 {
 	int i, err = 0;
@@ -1055,23 +1040,6 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
 		err = vm_map_pages_zero(vma, map->pages, map->count);
 		if (err)
 			goto out_put_map;
-	} else {
-#ifdef CONFIG_X86
-		/*
-		 * If the PTEs were not made special by the grant map
-		 * hypercall, do so here.
-		 *
-		 * This is racy since the mapping is already visible
-		 * to userspace but userspace should be well-behaved
-		 * enough to not touch it until the mmap() call
-		 * returns.
-		 */
-		if (!xen_feature(XENFEAT_gnttab_map_avail_bits)) {
-			apply_to_page_range(vma->vm_mm, vma->vm_start,
-					    vma->vm_end - vma->vm_start,
-					    set_grant_ptes_as_special, NULL);
-		}
-#endif
 	}
 
 	return 0;
-- 
2.26.2

Re: [PATCH v2 0/3] xen: remove some checks for always present Xen features

[Boris Ostrovsky]

On 7/30/21 3:18 AM, Juergen Gross wrote:
> Some features of Xen can be assumed to be always present, so add a
> central check to verify this being true and remove the other checks.
>
> Juergen Gross (3):
>   xen: check required Xen features
>   xen: assume XENFEAT_mmu_pt_update_preserve_ad being set for pv guests
>   xen: assume XENFEAT_gnttab_map_avail_bits being set for pv guests
>
>  arch/x86/xen/enlighten_pv.c | 12 ++----------
>  arch/x86/xen/mmu_pv.c       |  4 ++--
>  drivers/xen/features.c      | 18 ++++++++++++++++++
>  drivers/xen/gntdev.c        | 36 ++----------------------------------
>  4 files changed, 24 insertions(+), 46 deletions(-)



Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>