Re: [PATCH v3 9/9] KVM: x86: Add XSAVE Support for Architectural LBRs

[Xu, Like]

On 2021/3/5 0:31, Sean Christopherson wrote:
> Paolo, any thoughts on how to keep supported_xss aligned with support_xcr0,
> without spreading the logic around too much?

From 58be4152ced441395dfc439f446c5ad53bd48576 Mon Sep 17 00:00:00 2001
From: Like Xu <like.xu@xxxxxxxxxxxxxxx>
Date: Thu, 4 Mar 2021 13:21:38 +0800
Subject: [PATCH] KVM: x86: Refine the matching and clearing logic for 
supported_xss

"The existing clearing of supported_xss here is pointless".
Let's refine the code path in this way: initialize the supported_xss
with the filter of KVM_SUPPORTED_XSS mask and update its value in
a bit clear manner (rather than bit setting).

Before:
(1) kvm_arch_hardware_setup
    if (boot_cpu_has(X86_FEATURE_XSAVES))
        rdmsrl(MSR_IA32_XSS, host_xss);
    if (!kvm_cpu_cap_has(X86_FEATURE_XSAVES))
        supported_xss = 0;
    else supported_xss &= host_xss;
(2) vmx_set_cpu_caps
    supported_xss = 0;
    if (!cpu_has_vmx_xsaves())
        kvm_cpu_cap_clear(X86_FEATURE_XSAVES);
    else set available bits to supported_xss

After:
(1) kvm_arch_init
    if (boot_cpu_has(X86_FEATURE_XSAVES))
        rdmsrl(MSR_IA32_XSS, host_xss);
        supported_xss = host_xss & KVM_SUPPORTED_XSS;
(2) vmx_set_cpu_caps
    if (!cpu_has_vmx_xsaves())
        kvm_cpu_cap_clear(X86_FEATURE_XSAVES);
        supported_xss = 0;
    else clear un-available bits for supported_xss

Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Original-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Like Xu <like.xu@xxxxxxxxxxxxxxx>
---
 arch/x86/kvm/vmx/vmx.c |  5 +++--
 arch/x86/kvm/x86.c     | 13 +++++++------
 2 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 4bc4bb49aaa9..8706323547c4 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7288,9 +7288,10 @@ static __init void vmx_set_cpu_caps(void)
         kvm_cpu_cap_set(X86_FEATURE_UMIP);

     /* CPUID 0xD.1 */
-    supported_xss = 0;
-    if (!cpu_has_vmx_xsaves())
+    if (!cpu_has_vmx_xsaves()) {
         kvm_cpu_cap_clear(X86_FEATURE_XSAVES);
+        supported_xss = 0;
+    }

     /* CPUID 0x80000001 */
     if (!cpu_has_vmx_rdtscp())
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d773836ceb7a..99cb62035bb2 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -205,6 +205,8 @@ static struct kvm_user_return_msrs __percpu 
*user_return_msrs;
                 | XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
                 | XFEATURE_MASK_PKRU)

+#define KVM_SUPPORTED_XSS     0
+
 u64 __read_mostly host_efer;
 EXPORT_SYMBOL_GPL(host_efer);

@@ -8046,6 +8048,11 @@ int kvm_arch_init(void *opaque)
         supported_xcr0 = host_xcr0 & KVM_SUPPORTED_XCR0;
     }

+    if (boot_cpu_has(X86_FEATURE_XSAVES)) {
+        rdmsrl(MSR_IA32_XSS, host_xss);
+        supported_xss = host_xss & KVM_SUPPORTED_XSS;
+    }
+
     if (pi_inject_timer == -1)
         pi_inject_timer = housekeeping_enabled(HK_FLAG_TIMER);
 #ifdef CONFIG_X86_64
@@ -10421,9 +10428,6 @@ int kvm_arch_hardware_setup(void *opaque)

     rdmsrl_safe(MSR_EFER, &host_efer);

-    if (boot_cpu_has(X86_FEATURE_XSAVES))
-        rdmsrl(MSR_IA32_XSS, host_xss);
-
     r = ops->hardware_setup();
     if (r != 0)
         return r;
@@ -10431,9 +10435,6 @@ int kvm_arch_hardware_setup(void *opaque)
     memcpy(&kvm_x86_ops, ops->runtime_ops, sizeof(kvm_x86_ops));
     kvm_ops_static_call_update();

-    if (!kvm_cpu_cap_has(X86_FEATURE_XSAVES))
-        supported_xss = 0;
-
 #define __kvm_cpu_cap_has(UNUSED_, f) kvm_cpu_cap_has(f)
     cr4_reserved_bits = __cr4_reserved_bits(__kvm_cpu_cap_has, UNUSED_);
 #undef __kvm_cpu_cap_has
--
2.29.2