[PATCH 0/6] KVM TSS cleanups and speedups

[PATCH 0/6] KVM TSS cleanups and speedups

[Andy Lutomirski]

The first four patches here are intended to be straightforward
cleanups and to make a better base for Thomas' GDT series.  They may
be a slight speedup, too, because they remove an STR instruction
from the VMX entry path.

The last two patches are a reasonably large speedup but need careful
review.

FWIW, I can see lots of additional easy-ish speedups here.  For example:

 - The GDT reload on VM exit isn't really needed at all.  Instead let's
   just change the kernel limit to 0xFFFF.  Doing that naively would
   waste memory, but doing it carefully on top of Thomas' series would
   be straightforward and almost free.

 - RDMSR from MSR_GS_BASE is totally pointless.

 - Once I or someone finishes the FSGSBASE series, we get a big speedup
   there.

 - The LDT reload code should be split up and optimized better, I think.

Andy Lutomirski (6):
  x86/asm: Define the kernel TSS limit in a macro
  x86/kvm/vmx: Don't fetch the TSS base from the GDT
  x86/kvm/vmx: Get rid of segment_base() on 64-bit kernels
  x86/kvm/vmx: Simplify segment_base()
  x86/asm/64: Drop __cacheline_aligned from struct x86_hw_tss
  x86/kvm/vmx: Defer TR reload after VM exit

 arch/x86/include/asm/desc.h      | 58 ++++++++++++++++++++++++++++++------
 arch/x86/include/asm/processor.h | 12 +++++++-
 arch/x86/kernel/ioport.c         |  5 ++++
 arch/x86/kernel/process.c        | 10 +++++++
 arch/x86/kvm/vmx.c               | 63 +++++++++++++++++-----------------------
 5 files changed, 102 insertions(+), 46 deletions(-)

-- 
2.9.3

[PATCH 1/6] x86/asm: Define the kernel TSS limit in a macro

[Andy Lutomirski]

Rather than open-coding the kernel TSS limit in set_tss_desc(), make
it a real macro near the TSS layout definition.

This is purely a cleanup.

Cc: Thomas Garnier <thgarnie@google.com>
Cc: Jim Mattson <jmattson@google.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/include/asm/desc.h      | 10 +---------
 arch/x86/include/asm/processor.h | 10 ++++++++++
 2 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
index 12080d87da3b..2e781bcc5e12 100644
--- a/arch/x86/include/asm/desc.h
+++ b/arch/x86/include/asm/desc.h
@@ -177,16 +177,8 @@ static inline void __set_tss_desc(unsigned cpu, unsigned int entry, void *addr)
 	struct desc_struct *d = get_cpu_gdt_table(cpu);
 	tss_desc tss;
 
-	/*
-	 * sizeof(unsigned long) coming from an extra "long" at the end
-	 * of the iobitmap. See tss_struct definition in processor.h
-	 *
-	 * -1? seg base+limit should be pointing to the address of the
-	 * last valid byte
-	 */
 	set_tssldt_descriptor(&tss, (unsigned long)addr, DESC_TSS,
-			      IO_BITMAP_OFFSET + IO_BITMAP_BYTES +
-			      sizeof(unsigned long) - 1);
+			      __KERNEL_TSS_LIMIT);
 	write_gdt_entry(d, entry, &tss, DESC_TSS);
 }
 
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index e6cfe7ba2d65..feb2ab95b8f6 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -342,6 +342,16 @@ struct tss_struct {
 
 DECLARE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss);
 
+/*
+ * sizeof(unsigned long) coming from an extra "long" at the end
+ * of the iobitmap.
+ *
+ * -1? seg base+limit should be pointing to the address of the
+ * last valid byte
+ */
+#define __KERNEL_TSS_LIMIT	\
+	(IO_BITMAP_OFFSET + IO_BITMAP_BYTES + sizeof(unsigned long) - 1)
+
 #ifdef CONFIG_X86_32
 DECLARE_PER_CPU(unsigned long, cpu_current_top_of_stack);
 #endif
-- 
2.9.3

[PATCH 2/6] x86/kvm/vmx: Don't fetch the TSS base from the GDT

[Andy Lutomirski]

The current CPU's TSS base is a foregone conclusion, so there's no need
to parse it out of the segment tables.  This should save a couple cycles
(as STR is surely microcoded and poorly optimized) but, more importantly,
it's a cleanup and it means that segment_base() will never be called on
64-bit kernels.

Cc: Thomas Garnier <thgarnie@google.com>
Cc: Jim Mattson <jmattson@google.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/kvm/vmx.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index a236decb81e4..46420aaf1684 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2088,13 +2088,6 @@ static unsigned long segment_base(u16 selector)
 	return v;
 }
 
-static inline unsigned long kvm_read_tr_base(void)
-{
-	u16 tr;
-	asm("str %0" : "=g"(tr));
-	return segment_base(tr);
-}
-
 static void vmx_save_host_state(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
@@ -2294,10 +2287,11 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 
 		/*
 		 * Linux uses per-cpu TSS and GDT, so set these when switching
-		 * processors.
+		 * processors.  See 22.2.4.
 		 */
-		vmcs_writel(HOST_TR_BASE, kvm_read_tr_base()); /* 22.2.4 */
-		vmcs_writel(HOST_GDTR_BASE, gdt->address);   /* 22.2.4 */
+		vmcs_writel(HOST_TR_BASE,
+			    (unsigned long)this_cpu_ptr(&cpu_tss));
+		vmcs_writel(HOST_GDTR_BASE, gdt->address);
 
 		rdmsrl(MSR_IA32_SYSENTER_ESP, sysenter_esp);
 		vmcs_writel(HOST_IA32_SYSENTER_ESP, sysenter_esp); /* 22.2.3 */
-- 
2.9.3

[PATCH 3/6] x86/kvm/vmx: Get rid of segment_base() on 64-bit kernels

[Andy Lutomirski]

It was a bit buggy (it didn't list all segment types that needed
64-bit fixups), but the bug was irrelevant because it wasn't called
in any interesting context on 64-bit kernels and was only used for
data segents on 32-bit kernels.

To avoid confusion, make it explicitly 32-bit only.

Cc: Thomas Garnier <thgarnie@google.com>
Cc: Jim Mattson <jmattson@google.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/kvm/vmx.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 46420aaf1684..b1810a0edec3 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2059,6 +2059,12 @@ static bool update_transition_efer(struct vcpu_vmx *vmx, int efer_offset)
 	}
 }
 
+#ifdef CONFIG_X86_32
+/*
+ * On 32-bit kernels, VM exits still load the FS and GS bases from the
+ * VMCS rather than the segment table.  KVM uses this helper to figure
+ * out the current bases to poke them into the VMCS before entry.
+ */
 static unsigned long segment_base(u16 selector)
 {
 	struct desc_ptr *gdt = this_cpu_ptr(&host_gdt);
@@ -2081,12 +2087,9 @@ static unsigned long segment_base(u16 selector)
 	}
 	d = (struct desc_struct *)(table_base + (selector & ~7));
 	v = get_desc_base(d);
-#ifdef CONFIG_X86_64
-       if (d->s == 0 && (d->type == 2 || d->type == 9 || d->type == 11))
-               v |= ((unsigned long)((struct ldttss_desc64 *)d)->base3) << 32;
-#endif
 	return v;
 }
+#endif
 
 static void vmx_save_host_state(struct kvm_vcpu *vcpu)
 {
-- 
2.9.3

[PATCH 4/6] x86/kvm/vmx: Simplify segment_base()

[Andy Lutomirski]

Use actual pointer types for pointers (instead of unsigned long) and
replace hardcoded constants with the appropriate self-documenting
macros.

The function is still a bit messy, but this seems a lot better than
before to me.

This is mostly borrowed from a patch by Thomas Garnier.

Cc: Thomas Garnier <thgarnie@google.com>
Cc: Jim Mattson <jmattson@google.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/kvm/vmx.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index b1810a0edec3..17ef11c64702 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2069,24 +2069,23 @@ static unsigned long segment_base(u16 selector)
 {
 	struct desc_ptr *gdt = this_cpu_ptr(&host_gdt);
 	struct desc_struct *d;
-	unsigned long table_base;
+	struct desc_struct *table;
 	unsigned long v;
 
-	if (!(selector & ~3))
+	if (!(selector & ~SEGMENT_RPL_MASK))
 		return 0;
 
-	table_base = gdt->address;
+	table = (struct desc_struct *)gdt->address;
 
-	if (selector & 4) {           /* from ldt */
+	if ((selector & SEGMENT_TI_MASK) == SEGMENT_LDT) {
 		u16 ldt_selector = kvm_read_ldt();
 
-		if (!(ldt_selector & ~3))
+		if (!(ldt_selector & ~SEGMENT_RPL_MASK))
 			return 0;
 
-		table_base = segment_base(ldt_selector);
+		table = (struct desc_struct *)segment_base(ldt_selector);
 	}
-	d = (struct desc_struct *)(table_base + (selector & ~7));
-	v = get_desc_base(d);
+	v = get_desc_base(&table[selector >> 3]);
 	return v;
 }
 #endif
-- 
2.9.3

[PATCH 5/6] x86/asm/64: Drop __cacheline_aligned from struct x86_hw_tss

[Andy Lutomirski]

Historically, the entire TSS + io bitmap structure was cacheline
aligned, but commit ca241c75037b ("x86: unify tss_struct") changed it
(presumably inadvertently) so that the fixed-layout hardware part is
cacheline-aligned and the io bitmap is after the padding.  This wastes
24 bytes (the hardware part should be 104 bytes, but this pads it to
128 bytes) and, serves no purpose, and causes sizeof(struct
x86_hw_tss) to have a confusing value.

Drop the pointless alignment.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/include/asm/processor.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index feb2ab95b8f6..f385eca5407a 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -304,7 +304,7 @@ struct x86_hw_tss {
 	u16			reserved5;
 	u16			io_bitmap_base;
 
-} __attribute__((packed)) ____cacheline_aligned;
+} __attribute__((packed));
 #endif
 
 /*
-- 
2.9.3

[PATCH 6/6] x86/kvm/vmx: Defer TR reload after VM exit

[Andy Lutomirski]

Intel's VMX is daft and resets the hidden TSS limit register to 0x67
on VMX reload, and the 0x67 is not configurable.  KVM currently
reloads TR using the LTR instruction on every exit, but this is quite
slow because LTR is serializing.

The 0x67 limit is entirely harmless unless ioperm() is in use, so
defer the reload until a task using ioperm() is actually running.

Here's some poorly done benchmarking using kvm-unit-tests:

Before:

cpuid 1313
vmcall 1195
mov_from_cr8 11
mov_to_cr8 17
inl_from_pmtimer 6770
inl_from_qemu 6856
inl_from_kernel 2435
outl_to_kernel 1402

After:

cpuid 1291
vmcall 1181
mov_from_cr8 11
mov_to_cr8 16
inl_from_pmtimer 6457
inl_from_qemu 6209
inl_from_kernel 2339
outl_to_kernel 1391

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/include/asm/desc.h | 48 +++++++++++++++++++++++++++++++++++++++++++++
 arch/x86/kernel/ioport.c    |  5 +++++
 arch/x86/kernel/process.c   | 10 ++++++++++
 arch/x86/kvm/vmx.c          | 23 +++++++++-------------
 4 files changed, 72 insertions(+), 14 deletions(-)

diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
index 2e781bcc5e12..1e173459767b 100644
--- a/arch/x86/include/asm/desc.h
+++ b/arch/x86/include/asm/desc.h
@@ -205,6 +205,54 @@ static inline void native_load_tr_desc(void)
 	asm volatile("ltr %w0"::"q" (GDT_ENTRY_TSS*8));
 }
 
+static inline void force_reload_TR(void)
+{
+	struct desc_struct *d = get_cpu_gdt_table(smp_processor_id());
+	tss_desc tss;
+
+	memcpy(&tss, &d[GDT_ENTRY_TSS], sizeof(tss_desc));
+
+	/*
+	 * LTR requires an available TSS, and the TSS is currently
+	 * busy.  Make it be available so that LTR will work.
+	 */
+	tss.type = DESC_TSS;
+	write_gdt_entry(d, GDT_ENTRY_TSS, &tss, DESC_TSS);
+
+	load_TR_desc();
+}
+
+DECLARE_PER_CPU(bool, need_tr_refresh);
+
+static inline void refresh_TR(void)
+{
+	DEBUG_LOCKS_WARN_ON(preemptible());
+
+	if (unlikely(this_cpu_read(need_tr_refresh))) {
+		force_reload_TR();
+		this_cpu_write(need_tr_refresh, false);
+	}
+}
+
+/*
+ * If you do something evil that corrupts the cached TSS limit (I'm looking
+ * at you, VMX exits), call this function.
+ *
+ * The optimization here is that the TSS limit only matters for Linux if the
+ * IO bitmap is in use.  If the TSS limit gets forced to its minimum value,
+ * everything works except that IO bitmap will be ignored and all CPL 3 IO
+ * instructions will #GP, which is exactly what we want for normal tasks.
+ */
+static inline void invalidate_tss_limit(void)
+{
+	DEBUG_LOCKS_WARN_ON(preemptible());
+
+	if (unlikely(test_thread_flag(TIF_IO_BITMAP)))
+		refresh_TR();
+	else
+		this_cpu_write(need_tr_refresh, true);
+}
+
 static inline void native_load_gdt(const struct desc_ptr *dtr)
 {
 	asm volatile("lgdt %0"::"m" (*dtr));
diff --git a/arch/x86/kernel/ioport.c b/arch/x86/kernel/ioport.c
index 589b3193f102..b01bc8517450 100644
--- a/arch/x86/kernel/ioport.c
+++ b/arch/x86/kernel/ioport.c
@@ -16,6 +16,7 @@
 #include <linux/syscalls.h>
 #include <linux/bitmap.h>
 #include <asm/syscalls.h>
+#include <asm/desc.h>
 
 /*
  * this changes the io permissions bitmap in the current task.
@@ -45,6 +46,10 @@ asmlinkage long sys_ioperm(unsigned long from, unsigned long num, int turn_on)
 		memset(bitmap, 0xff, IO_BITMAP_BYTES);
 		t->io_bitmap_ptr = bitmap;
 		set_thread_flag(TIF_IO_BITMAP);
+
+		preempt_disable();
+		refresh_TR();
+		preempt_enable();
 	}
 
 	/*
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index b615a1113f58..7780efa635b9 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -32,6 +32,7 @@
 #include <asm/mce.h>
 #include <asm/vm86.h>
 #include <asm/switch_to.h>
+#include <asm/desc.h>
 
 /*
  * per-CPU TSS segments. Threads are completely 'soft' on Linux,
@@ -64,6 +65,9 @@ __visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss) = {
 };
 EXPORT_PER_CPU_SYMBOL(cpu_tss);
 
+DEFINE_PER_CPU(bool, need_tr_refresh);
+EXPORT_PER_CPU_SYMBOL_GPL(need_tr_refresh);
+
 /*
  * this gets called so that we can store lazy state into memory and copy the
  * current task into the new thread.
@@ -209,6 +213,12 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
 		 */
 		memcpy(tss->io_bitmap, next->io_bitmap_ptr,
 		       max(prev->io_bitmap_max, next->io_bitmap_max));
+
+		/*
+		 * Make sure that the TSS limit is correct for the CPU
+		 * to notice the IO bitmap.
+		 */
+		refresh_TR();
 	} else if (test_tsk_thread_flag(prev_p, TIF_IO_BITMAP)) {
 		/*
 		 * Clear any possible leftover bits:
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 17ef11c64702..733c8a3c912e 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -1992,19 +1992,6 @@ static void add_atomic_switch_msr(struct vcpu_vmx *vmx, unsigned msr,
 	m->host[i].value = host_val;
 }
 
-static void reload_tss(void)
-{
-	/*
-	 * VT restores TR but not its size.  Useless.
-	 */
-	struct desc_ptr *gdt = this_cpu_ptr(&host_gdt);
-	struct desc_struct *descs;
-
-	descs = (void *)gdt->address;
-	descs[GDT_ENTRY_TSS].type = 9; /* available TSS */
-	load_TR_desc();
-}
-
 static bool update_transition_efer(struct vcpu_vmx *vmx, int efer_offset)
 {
 	u64 guest_efer = vmx->vcpu.arch.efer;
@@ -2174,7 +2161,7 @@ static void __vmx_load_host_state(struct vcpu_vmx *vmx)
 		loadsegment(es, vmx->host_state.es_sel);
 	}
 #endif
-	reload_tss();
+	invalidate_tss_limit();
 #ifdef CONFIG_X86_64
 	wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_host_kernel_gs_base);
 #endif
@@ -2295,6 +2282,14 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 			    (unsigned long)this_cpu_ptr(&cpu_tss));
 		vmcs_writel(HOST_GDTR_BASE, gdt->address);
 
+		/*
+		 * VM exits change the host TR limit to 0x67 after a VM
+		 * exit.  This is okay, since 0x67 covers everything except
+		 * the IO bitmap and have have code to handle the IO bitmap
+		 * being lost after a VM exit.
+		 */
+		BUILD_BUG_ON(IO_BITMAP_OFFSET - 1 != 0x67);
+
 		rdmsrl(MSR_IA32_SYSENTER_ESP, sysenter_esp);
 		vmcs_writel(HOST_IA32_SYSENTER_ESP, sysenter_esp); /* 22.2.3 */
 
-- 
2.9.3

Re: [PATCH 4/6] x86/kvm/vmx: Simplify segment_base()

[Thomas Garnier]

On Mon, Feb 20, 2017 at 8:56 AM, Andy Lutomirski <luto@kernel.org> wrote:
> Use actual pointer types for pointers (instead of unsigned long) and
> replace hardcoded constants with the appropriate self-documenting
> macros.
>
> The function is still a bit messy, but this seems a lot better than
> before to me.
>
> This is mostly borrowed from a patch by Thomas Garnier.
>

Looks good, I like that the 64-bit usage could be removed. Thanks!

> Cc: Thomas Garnier <thgarnie@google.com>
> Cc: Jim Mattson <jmattson@google.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
>  arch/x86/kvm/vmx.c | 15 +++++++--------
>  1 file changed, 7 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index b1810a0edec3..17ef11c64702 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -2069,24 +2069,23 @@ static unsigned long segment_base(u16 selector)
>  {
>         struct desc_ptr *gdt = this_cpu_ptr(&host_gdt);
>         struct desc_struct *d;
> -       unsigned long table_base;
> +       struct desc_struct *table;
>         unsigned long v;
>
> -       if (!(selector & ~3))
> +       if (!(selector & ~SEGMENT_RPL_MASK))
>                 return 0;
>
> -       table_base = gdt->address;
> +       table = (struct desc_struct *)gdt->address;
>
> -       if (selector & 4) {           /* from ldt */
> +       if ((selector & SEGMENT_TI_MASK) == SEGMENT_LDT) {
>                 u16 ldt_selector = kvm_read_ldt();
>
> -               if (!(ldt_selector & ~3))
> +               if (!(ldt_selector & ~SEGMENT_RPL_MASK))
>                         return 0;
>
> -               table_base = segment_base(ldt_selector);
> +               table = (struct desc_struct *)segment_base(ldt_selector);
>         }
> -       d = (struct desc_struct *)(table_base + (selector & ~7));
> -       v = get_desc_base(d);
> +       v = get_desc_base(&table[selector >> 3]);
>         return v;
>  }
>  #endif
> --
> 2.9.3
>



-- 
Thomas

Re: [PATCH 6/6] x86/kvm/vmx: Defer TR reload after VM exit

[Paolo Bonzini]

On 20/02/2017 17:56, Andy Lutomirski wrote:
> +{
> +	DEBUG_LOCKS_WARN_ON(preemptible());
> +
> +	if (unlikely(this_cpu_read(need_tr_refresh))) {
> +		force_reload_TR();
> +		this_cpu_write(need_tr_refresh, false);
> +	}
> +}
> +
> +/*
> + * If you do something evil that corrupts the cached TSS limit (I'm looking
> + * at you, VMX exits), call this function.
> + *
> + * The optimization here is that the TSS limit only matters for Linux if the
> + * IO bitmap is in use.  If the TSS limit gets forced to its minimum value,
> + * everything works except that IO bitmap will be ignored and all CPL 3 IO
> + * instructions will #GP, which is exactly what we want for normal tasks.
> + */
> +static inline void invalidate_tss_limit(void)
> +{
> +	DEBUG_LOCKS_WARN_ON(preemptible());
> +
> +	if (unlikely(test_thread_flag(TIF_IO_BITMAP)))
> +		refresh_TR();

This must be force_reload_TR, not refresh_TR.

Paolo

> +	else
> +		this_cpu_write(need_tr_refresh, true);

Re: [PATCH 0/6] KVM TSS cleanups and speedups

[Paolo Bonzini]

On 20/02/2017 17:56, Andy Lutomirski wrote:
> The first four patches here are intended to be straightforward
> cleanups and to make a better base for Thomas' GDT series.  They may
> be a slight speedup, too, because they remove an STR instruction
> from the VMX entry path.
> 
> The last two patches are a reasonably large speedup but need careful
> review.

I spotted (I think) a bug in patch 6, but apart from that it looks nice.

Paolo

> FWIW, I can see lots of additional easy-ish speedups here.  For example:
> 
>  - The GDT reload on VM exit isn't really needed at all.  Instead let's
>    just change the kernel limit to 0xFFFF.  Doing that naively would
>    waste memory, but doing it carefully on top of Thomas' series would
>    be straightforward and almost free.
> 
>  - RDMSR from MSR_GS_BASE is totally pointless.
> 
>  - Once I or someone finishes the FSGSBASE series, we get a big speedup
>    there.
> 
>  - The LDT reload code should be split up and optimized better, I think.
> 
> Andy Lutomirski (6):
>   x86/asm: Define the kernel TSS limit in a macro
>   x86/kvm/vmx: Don't fetch the TSS base from the GDT
>   x86/kvm/vmx: Get rid of segment_base() on 64-bit kernels
>   x86/kvm/vmx: Simplify segment_base()
>   x86/asm/64: Drop __cacheline_aligned from struct x86_hw_tss
>   x86/kvm/vmx: Defer TR reload after VM exit
> 
>  arch/x86/include/asm/desc.h      | 58 ++++++++++++++++++++++++++++++------
>  arch/x86/include/asm/processor.h | 12 +++++++-
>  arch/x86/kernel/ioport.c         |  5 ++++
>  arch/x86/kernel/process.c        | 10 +++++++
>  arch/x86/kvm/vmx.c               | 63 +++++++++++++++++-----------------------
>  5 files changed, 102 insertions(+), 46 deletions(-)
>