* [PATCH] KVM: selftest: Move XFD CPUID checking out of __vm_xsave_require_permission()
@ 2022-11-25 2:38 Wang, Lei
2022-11-28 17:03 ` Sean Christopherson
0 siblings, 1 reply; 3+ messages in thread
From: Wang, Lei @ 2022-11-25 2:38 UTC (permalink / raw)
To: kvm; +Cc: pbonzini, seanjc, yang.zhong
kvm_cpu_has(X86_FEATURE_XFD) will call kvm_get_supported_cpuid() which will
cache the cpuid information when it is firstly called. Move this line out
of __vm_xsave_require_permission() and check it afterwards so that the
CPUID change will not be veiled by the cached CPUID information.
Signed-off-by: Wang, Lei <lei4.wang@intel.com>
---
tools/testing/selftests/kvm/lib/x86_64/processor.c | 2 --
tools/testing/selftests/kvm/x86_64/amx_test.c | 1 +
2 files changed, 1 insertion(+), 2 deletions(-)
diff --git a/tools/testing/selftests/kvm/lib/x86_64/processor.c b/tools/testing/selftests/kvm/lib/x86_64/processor.c
index 39c4409ef56a..5686eacd4700 100644
--- a/tools/testing/selftests/kvm/lib/x86_64/processor.c
+++ b/tools/testing/selftests/kvm/lib/x86_64/processor.c
@@ -616,8 +616,6 @@ void __vm_xsave_require_permission(int bit, const char *name)
.addr = (unsigned long) &bitmask
};
- TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_XFD));
-
kvm_fd = open_kvm_dev_path_or_exit();
rc = __kvm_ioctl(kvm_fd, KVM_GET_DEVICE_ATTR, &attr);
close(kvm_fd);
diff --git a/tools/testing/selftests/kvm/x86_64/amx_test.c b/tools/testing/selftests/kvm/x86_64/amx_test.c
index dadcbad10a1d..1e3457ff304b 100644
--- a/tools/testing/selftests/kvm/x86_64/amx_test.c
+++ b/tools/testing/selftests/kvm/x86_64/amx_test.c
@@ -312,6 +312,7 @@ int main(int argc, char *argv[])
/* Create VM */
vm = vm_create_with_one_vcpu(&vcpu, guest_code);
+ TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_XFD));
TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_XSAVE));
TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_AMX_TILE));
TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_XTILECFG));
--
2.34.1
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] KVM: selftest: Move XFD CPUID checking out of __vm_xsave_require_permission()
2022-11-25 2:38 [PATCH] KVM: selftest: Move XFD CPUID checking out of __vm_xsave_require_permission() Wang, Lei
@ 2022-11-28 17:03 ` Sean Christopherson
2022-11-29 1:01 ` Wang, Lei
0 siblings, 1 reply; 3+ messages in thread
From: Sean Christopherson @ 2022-11-28 17:03 UTC (permalink / raw)
To: Wang, Lei; +Cc: kvm, pbonzini, yang.zhong
On Thu, Nov 24, 2022, Wang, Lei wrote:
> kvm_cpu_has(X86_FEATURE_XFD) will call kvm_get_supported_cpuid() which will
> cache the cpuid information when it is firstly called. Move this line out
> of __vm_xsave_require_permission() and check it afterwards so that the
> CPUID change will not be veiled by the cached CPUID information.
Please call out exactly what CPUID change is being referred to. Someone that
doesn't already know about ARCH_REQ_XCOMP_GUEST_PERM and it's interaction with
KVM_GET_SUPPORTED_CPUID will have zero clue what this fixes.
E.g.
Move the kvm_cpu_has() check on X86_FEATURE_XFD out of the helper to
enable off-by-default XSAVE-managed features and into the one test that
currenty requires XFD (XFeature Disable) support. kvm_cpu_has() uses
kvm_get_supported_cpuid() and thus caches KVM_GET_SUPPORTED_CPUID, and so
using kvm_cpu_has() before ARCH_REQ_XCOMP_GUEST_PERM effectively results
in the test caching stale values, e.g. subsequent checks on AMX_TILE will
get false negatives.
Although off-by-default features are nonsensical without XFD, checking
for XFD virtualization prior to enabling such features isn't strictly
required.
Fixes: 7fbb653e01fd ("KVM: selftests: Check KVM's supported CPUID, not host CPUID, for XFD")
> Signed-off-by: Wang, Lei <lei4.wang@intel.com>
> ---
> tools/testing/selftests/kvm/lib/x86_64/processor.c | 2 --
> tools/testing/selftests/kvm/x86_64/amx_test.c | 1 +
> 2 files changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/lib/x86_64/processor.c b/tools/testing/selftests/kvm/lib/x86_64/processor.c
> index 39c4409ef56a..5686eacd4700 100644
> --- a/tools/testing/selftests/kvm/lib/x86_64/processor.c
> +++ b/tools/testing/selftests/kvm/lib/x86_64/processor.c
> @@ -616,8 +616,6 @@ void __vm_xsave_require_permission(int bit, const char *name)
> .addr = (unsigned long) &bitmask
> };
>
> - TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_XFD));
> -
> kvm_fd = open_kvm_dev_path_or_exit();
> rc = __kvm_ioctl(kvm_fd, KVM_GET_DEVICE_ATTR, &attr);
> close(kvm_fd);
> diff --git a/tools/testing/selftests/kvm/x86_64/amx_test.c b/tools/testing/selftests/kvm/x86_64/amx_test.c
> index dadcbad10a1d..1e3457ff304b 100644
> --- a/tools/testing/selftests/kvm/x86_64/amx_test.c
> +++ b/tools/testing/selftests/kvm/x86_64/amx_test.c
> @@ -312,6 +312,7 @@ int main(int argc, char *argv[])
> /* Create VM */
> vm = vm_create_with_one_vcpu(&vcpu, guest_code);
>
> + TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_XFD));
I think we should disallow kvm_get_supported_cpuid() before
__vm_xsave_require_permission(), otherwise we'll reintroduce a similar bug in the
future.
> TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_XSAVE));
> TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_AMX_TILE));
> TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_XTILECFG));
And then as a follow-up, we should move these above vm_create_with_one_vcpu(),
checking them after vm_create_with_one_vcpu() is odd.
I'll send a v2 with the reworded changelog and additional patches to assert that
__vm_xsave_require_permission() isn't used after kvm_get_supported_cpuid().
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: [PATCH] KVM: selftest: Move XFD CPUID checking out of __vm_xsave_require_permission()
2022-11-28 17:03 ` Sean Christopherson
@ 2022-11-29 1:01 ` Wang, Lei
0 siblings, 0 replies; 3+ messages in thread
From: Wang, Lei @ 2022-11-29 1:01 UTC (permalink / raw)
To: Sean Christopherson; +Cc: kvm, pbonzini, yang.zhong
On 11/29/2022 1:03 AM, Sean Christopherson wrote:
> On Thu, Nov 24, 2022, Wang, Lei wrote:
>> kvm_cpu_has(X86_FEATURE_XFD) will call kvm_get_supported_cpuid() which will
>> cache the cpuid information when it is firstly called. Move this line out
>> of __vm_xsave_require_permission() and check it afterwards so that the
>> CPUID change will not be veiled by the cached CPUID information.
>
> Please call out exactly what CPUID change is being referred to. Someone that
> doesn't already know about ARCH_REQ_XCOMP_GUEST_PERM and it's interaction with
> KVM_GET_SUPPORTED_CPUID will have zero clue what this fixes.
>
> E.g.
>
> Move the kvm_cpu_has() check on X86_FEATURE_XFD out of the helper to
> enable off-by-default XSAVE-managed features and into the one test that
> currenty requires XFD (XFeature Disable) support. kvm_cpu_has() uses
> kvm_get_supported_cpuid() and thus caches KVM_GET_SUPPORTED_CPUID, and so
> using kvm_cpu_has() before ARCH_REQ_XCOMP_GUEST_PERM effectively results
> in the test caching stale values, e.g. subsequent checks on AMX_TILE will
> get false negatives.
>
> Although off-by-default features are nonsensical without XFD, checking
> for XFD virtualization prior to enabling such features isn't strictly
> required.
>
> Fixes: 7fbb653e01fd ("KVM: selftests: Check KVM's supported CPUID, not host CPUID, for XFD")
Makes sense.
>> Signed-off-by: Wang, Lei <lei4.wang@intel.com>
>> ---
>> tools/testing/selftests/kvm/lib/x86_64/processor.c | 2 --
>> tools/testing/selftests/kvm/x86_64/amx_test.c | 1 +
>> 2 files changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/tools/testing/selftests/kvm/lib/x86_64/processor.c b/tools/testing/selftests/kvm/lib/x86_64/processor.c
>> index 39c4409ef56a..5686eacd4700 100644
>> --- a/tools/testing/selftests/kvm/lib/x86_64/processor.c
>> +++ b/tools/testing/selftests/kvm/lib/x86_64/processor.c
>> @@ -616,8 +616,6 @@ void __vm_xsave_require_permission(int bit, const char *name)
>> .addr = (unsigned long) &bitmask
>> };
>>
>> - TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_XFD));
>> -
>> kvm_fd = open_kvm_dev_path_or_exit();
>> rc = __kvm_ioctl(kvm_fd, KVM_GET_DEVICE_ATTR, &attr);
>> close(kvm_fd);
>> diff --git a/tools/testing/selftests/kvm/x86_64/amx_test.c b/tools/testing/selftests/kvm/x86_64/amx_test.c
>> index dadcbad10a1d..1e3457ff304b 100644
>> --- a/tools/testing/selftests/kvm/x86_64/amx_test.c
>> +++ b/tools/testing/selftests/kvm/x86_64/amx_test.c
>> @@ -312,6 +312,7 @@ int main(int argc, char *argv[])
>> /* Create VM */
>> vm = vm_create_with_one_vcpu(&vcpu, guest_code);
>>
>> + TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_XFD));
>
> I think we should disallow kvm_get_supported_cpuid() before
> __vm_xsave_require_permission(), otherwise we'll reintroduce a similar bug in the
> future.
Agree.
>> TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_XSAVE));
>> TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_AMX_TILE));
>> TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_XTILECFG));
>
> And then as a follow-up, we should move these above vm_create_with_one_vcpu(),
> checking them after vm_create_with_one_vcpu() is odd.
>
> I'll send a v2 with the reworded changelog and additional patches to assert that
> __vm_xsave_require_permission() isn't used after kvm_get_supported_cpuid().
Already seen your patches, thanks Sean!
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2022-11-29 1:01 UTC | newest]
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2022-11-25 2:38 [PATCH] KVM: selftest: Move XFD CPUID checking out of __vm_xsave_require_permission() Wang, Lei
2022-11-28 17:03 ` Sean Christopherson
2022-11-29 1:01 ` Wang, Lei
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