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X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: qTUMYPXhALpt84PgT9kfuezhMk8MVfJ9ONK1dBDmSfjvSqbr099+T1OkpcvabEJ0XwCxWxFJL5p6xtC/2G8n5csI8qH7vNHBLDsAg11v+nQ= X-MS-Exchange-Transport-CrossTenantHeadersStamped: MW5PR10MB5875 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1099,Hydra:6.1.7,FMLib:17.12.80.40 definitions=2025-06-26_06,2025-06-26_05,2025-03-28_01 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 mlxscore=0 bulkscore=0 spamscore=0 phishscore=0 adultscore=0 malwarescore=0 suspectscore=0 mlxlogscore=999 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2505160000 definitions=main-2506260164 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwNjI2MDE2NCBTYWx0ZWRfX07phhGJUoYfp T4slW3FkP8ZVkFv9hAlV5HTauJaM28ogDMmpeKlmoo2KXFLC17aZkASbqe7lRePrsD+qxodfWzd XY2M+Mymaz8UajUP7p+VzquMq2Ls5fCgnuXXCz5Qo4Q0LlS2X4tdfy8zbbDYHI2/lrkQOUTyckP v3s2Zkscq1V5As3bbqE6woNZIAyK1271yXsoqAo2HSg++QD16Ay63C1ogp+EENVI/6SBx7/mTzb unjXUbg1h4RWcmDcV20urcinejfQTGanU7sJwVCHQ3ROoOxhc6A/ffFoe6trDDA/FZV/LCpfMKb 0jDSgZgB8xN7tbqe4PsXefEOZjGP4WeoJGEluiW60gSYBU1uJCMKN0F8WzERtPli2RyfCe7toyR PCdbWRoU+iMf35xYthThV/xD87TqlQxueJWGGUQUTucpU+ORiCQK9oQ7z+xkCvr0bHRXTfq2 X-Proofpoint-GUID: -97sXIdkq7RfneUb4R66f6jzb6qcsa2O X-Authority-Analysis: v=2.4 cv=QNpoRhLL c=1 sm=1 tr=0 ts=685d9de7 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=IkcTkHD0fZMA:10 a=6IFa9wvqVegA:10 a=GoEa3M9JfhUA:10 a=VwQbUJbxAAAA:8 a=QyXUC8HyAAAA:8 a=yPCof4ZbAAAA:8 a=QH-YIIke1X8NFttTr18A:9 a=QEXdDO2ut3YA:10 cc=ntf awl=host:14723 X-Proofpoint-ORIG-GUID: -97sXIdkq7RfneUb4R66f6jzb6qcsa2O On 6/26/25 16:02, 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 understand, no one likes to break ABI. However one can argue that any AMD code (and even Intel) is supposed to work without ARCH_CAPABILITIES (AMD cpus never have this capability and some Intel cpus don't either). Also if code running on AMD rely on ARCH_CAPABILITIES then it's probably wrong. We can also imagine that exposing this capability can induce incorrect behaviors in the guest like "the ARCH_CAPABILITIES is present so that's an Intel cpu". >> 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. >> >> --- >> arch/x86/kvm/svm/svm.c | 3 +++ >> 1 file changed, 3 insertions(+) >> >> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c >> index ab9b947dbf4f..600d2029156e 100644 >> --- a/arch/x86/kvm/svm/svm.c >> +++ b/arch/x86/kvm/svm/svm.c >> @@ -5469,6 +5469,9 @@ static __init void svm_set_cpu_caps(void) >> >> /* Don't advertise Bus Lock Detect to guest if SVM support is absent */ >> kvm_cpu_cap_clear(X86_FEATURE_BUS_LOCK_DETECT); >> + >> + /* Don't advertise ARCH_CAPABILITIES on AMD */ >> + kvm_cpu_cap_clear(X86_FEATURE_ARCH_CAPABILITIES); > > Strictly speaking, I think we'd want to update svm_has_emulated_msr() as well. > Yes, that would be cleaner. even though the access to the MSR is prevented by KVM when the ARCH_CAPABILITIES is cleared. Thanks, alex. >> } >> >> static __init int svm_hardware_setup(void) >> -- >> 2.43.5 >>