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classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2505160000 definitions=main-2506270170 X-Authority-Analysis: v=2.4 cv=a8gw9VSF c=1 sm=1 tr=0 ts=685f05f7 b=1 cx=c_pps a=e1sVV491RgrpLwSTMOnk8w==:117 a=e1sVV491RgrpLwSTMOnk8w==:17 a=6eWqkTHjU83fiwn7nKZWdM+Sl24=:19 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=wKuvFiaSGQ0qltdbU6+NXLB8nM8=:19 a=Ol13hO9ccFRV9qXi2t6ftBPywas=:19 a=xqWC_Br6kY4A:10 a=8nJEP1OIZ-IA:10 a=6IFa9wvqVegA:10 a=GoEa3M9JfhUA:10 a=VwQbUJbxAAAA:8 a=yPCof4ZbAAAA:8 a=QyXUC8HyAAAA:8 a=CM8ZFXFThYBCFr_Gz0EA:9 a=3ZKOabzyN94A:10 a=wPNLvfGTeEIA:10 cc=ntf awl=host:14723 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwNjI3MDE3MCBTYWx0ZWRfX6UjYtH0SlptZ Z8S0W1F/+VuXjhTBlgPWB5oUOyH2lU2mawDU6nxfQzwVj6h5deJDNPyYN3svMnlrHSgUa42FXje KupEPbnj41Llcy7VbwaFIkiZqUR8vgeofwH0zwGNE+9djfLKzWylpTPj+ZMCLMi7SQCWV39NyZ5 O4l4YdpStD4r9y+fu3PU9ChNXuYoBqCyUI5vJdDhtHGeeFMfLiyq76MUxQzCJrZ9gLAke17qA6X eo31RU7kllr9cIG4InL0yc78ofH9SuesNgy1cG4TP8VJRUC2gPMXEM9pIS9dg9l/v5tw+JISlL8 LB318E5Deg3C4AVqIgJDRKRdm21fLnsNwhJVn4rS5yHjagfKUDy+HT1ZCGGWmfgEqY028c3CwDw g17LNLSimgZk1u9L88yrcNkwBeWdIlMqcHS4XayY7/hAzny7v24wVCQzFEdIzxgtZvzj8jU7 X-Proofpoint-GUID: Fcz37P5AccWB0EBgXfRf9OlTDGIj9gk4 X-Proofpoint-ORIG-GUID: Fcz37P5AccWB0EBgXfRf9OlTDGIj9gk4 On Fri, Jun 27, 2025 at 08:23:52AM +0200, Alexandre Chartre wrote: > > On 6/27/25 07:41, Xiaoyao Li wrote: > > On 6/26/2025 10:02 PM, Sean Christopherson wrote: > > > +Jim > > > > > > For the 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. Hi Sean, Part of the virtualization stack is to lie accurately and in this case KVM is doing it incorrectly. Not fixing it b/c of it being for 7 years in and being part of an ABI but saying it should be fixed in QEMU sounds like you agree technically, but are constrained by a policy. N.B. Also the TSC deadline MSR is advertised yet AMD does not support it. Looping in Linus here. Linus, thoughts? > > 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. >