Re: [PATCH] KVM: x86/cpuid: Clear XFD for component i if the base feature is missing

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On 18/1/2022 1:31 am, Paolo Bonzini wrote:
On 1/17/22 08:45, Like Xu wrote:
From: Like Xu <likexu@xxxxxxxxxxx>

According to Intel extended feature disable (XFD) spec, the sub-function i
(i > 1) of CPUID function 0DH enumerates "details for state component i.
ECX[2] enumerates support for XFD support for this state component."

If KVM does not report F(XFD) feature (e.g. due to CONFIG_X86_64),
then the corresponding XFD support for any state component i
should also be removed. Translate this dependency into KVM terms.

Fixes: 690a757d610e ("kvm: x86: Add CPUID support for Intel AMX")
Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
---
  arch/x86/kvm/cpuid.c | 3 +++
  1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index c55e57b30e81..e96efef4f048 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -886,6 +886,9 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
                  --array->nent;
                  continue;
              }
+
+            if (!kvm_cpu_cap_has(X86_FEATURE_XFD))
+                entry->ecx &= ~BIT_ULL(2);
              entry->edx = 0;
          }
          break;

Generally this is something that is left to userspace.  Apart from the usecase of "call KVM_GET_SUPPORTED_CPUID and pass it to KVM_SET_CPUID2", userspace should know what any changed bits mean.

Paolo


I totally agree that setting the appropriate CPUID bits for a feature is a user space thing.

But this patch is more focused on fixing a different type of problem, which is
that the capabilities exposed by KVM should not *contradict each other* :

	a user space may be confused with the current code base :
	- why KVM does not have F(XFD) feature (MSR_IA32_XFD and XFD_ERR non-exit),
	- but KVM reports XFD support for state component i individually;

This is like KVM reporting PEBS when no PMU capacity is reported (due to module param).

Does this clarification help ?

Thanks,
Like Xu




[Index of Archives]     [KVM ARM]     [KVM ia64]     [KVM ppc]     [Virtualization Tools]     [Spice Development]     [Libvirt]     [Libvirt Users]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite Questions]     [Linux Kernel]     [Linux SCSI]     [XFree86]

  Powered by Linux