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scope, "KVM: x86:" >> >> On Thu, Jun 26, 2025, Alexandre Chartre wrote: >>> KVM emulates the ARCH_CAPABILITIES on x86 for both vmx and svm. >>> However the IA32_ARCH_CAPABILITIES MSR is an Intel-specific MSR >>> so it makes no sense to emulate it on AMD. >>> >>> The AMD documentation specifies that this MSR is not defined on >>> the AMD architecture. So emulating this MSR on AMD can even cause >>> issues (like Windows BSOD) as the guest OS might not expect this >>> MSR to exist on such architecture. >>> >>> Signed-off-by: Alexandre Chartre >>> --- >>> >>> A similar patch was submitted some years ago but it looks like it felt >>> through the cracks: >>> https://lore.kernel.org/kvm/20190307093143.77182-1- xiaoyao.li@linux.intel.com/ >> It didn't fall through the cracks, we deliberately elected to emulate the MSR in >> common code so that KVM's advertised CPUID support would match KVM's emulation. >> >>    On Thu, 2019-03-07 at 19:15 +0100, Paolo Bonzini wrote: >>    > On 07/03/19 18:37, Sean Christopherson wrote: >>    > > On Thu, Mar 07, 2019 at 05:31:43PM +0800, Xiaoyao Li wrote: >>    > > > At present, we report F(ARCH_CAPABILITIES) for x86 arch(both vmx and svm) >>    > > > unconditionally, but we only emulate this MSR in vmx. It will cause #GP >>    > > > while guest kernel rdmsr(MSR_IA32_ARCH_CAPABILITIES) in an AMD host. >>    > > > >>    > > > Since MSR IA32_ARCH_CAPABILITIES is an intel-specific MSR, it makes no >>    > > > sense to emulate it in svm. Thus this patch chooses to only emulate it >>    > > > for vmx, and moves the related handling to vmx related files. >>    > > >>    > > What about emulating the MSR on an AMD host for testing purpsoes?  It >>    > > might be a useful way for someone without Intel hardware to test spectre >>    > > related flows. >>    > > >>    > > In other words, an alternative to restricting emulation of the MSR to >>    > > Intel CPUS would be to move MSR_IA32_ARCH_CAPABILITIES handling into >>    > > kvm_{get,set}_msr_common().  Guest access to MSR_IA32_ARCH_CAPABILITIES >>    > > is gated by X86_FEATURE_ARCH_CAPABILITIES in the guest's CPUID, e.g. >>    > > RDMSR will naturally #GP fault if userspace passes through the host's >>    > > CPUID on a non-Intel system. >>    > >>    > This is also better because it wouldn't change the guest ABI for AMD >>    > processors.  Dropping CPUID flags is generally not a good idea. >>    > >>    > Paolo >> >> I don't necessarily disagree about emulating ARCH_CAPABILITIES being pointless, >> but Paolo's point about not changing ABI for existing setups still stands.  This >> has been KVM's behavior for 6 years (since commit 0cf9135b773b ("KVM: x86: Emulate >> MSR_IA32_ARCH_CAPABILITIES on AMD hosts"); 7 years, if we go back to when KVM >> enumerated support without emulating the MSR (commit 1eaafe91a0df ("kvm: x86: >> IA32_ARCH_CAPABILITIES is always supported"). >> >> And it's not like KVM is forcing userspace to enumerate support for >> ARCH_CAPABILITIES, e.g. QEMU's named AMD configs don't enumerate support.  So >> while I completely agree KVM's behavior is odd and annoying for userspace to deal >> with, this is probably something that should be addressed in userspace. >> >>> I am resurecting this change because some recent Windows updates (like OS Build >>> 26100.4351) crashes on AMD KVM guests (BSOD with Stop code: UNSUPPORTED PROCESSOR) >>> just because the ARCH_CAPABILITIES is available. > > Isn't it the Windows bugs? I think it is incorrect to assume AMD will never implement ARCH_CAPABILITIES. > Yes, although on one hand they are just following the current AMD specification which says that ARCH_CAPABILITIES is not defined on AMD cpus; but on the other hand they are breaking a 6+ years behavior. So it might be nice if we could prevent such an issue in the future. Note that a Windows update preview has just been released with a fix (OS Build 26100.4484), but the Windows automatic update will still install the version with the issue at the moment (automatic update doesn't install preview). alex.