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SubjectRe: [PATCH] KVM: x86: Always enable legacy fp/sse
On Tue, Aug 16, 2022, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
>
> A live migration under qemu is currently failing when the source
> host is ~Nehalem era (pre-xsave) and the destination is much newer,
> (configured with a guest CPU type of Nehalem).
> QEMU always calls kvm_put_xsave, even on this combination because
> KVM_CAP_CHECK_EXTENSION_VM always returns true for KVM_CAP_XSAVE.
>
> When QEMU calls kvm_put_xsave it's rejected by
> fpu_copy_uabi_to_guest_fpstate->
> copy_uabi_to_xstate->
> validate_user_xstate_header
>
> when the validate checks the loaded xfeatures against
> user_xfeatures, which it finds to be 0.
>
> I think our initialisation of user_xfeatures is being
> too strict here, and we should always allow the base FP/SSE.
>
> Fixes: ad856280ddea ("x86/kvm/fpu: Limit guest user_xfeatures to supported bits of XCR0")
> bz: https://bugzilla.redhat.com/show_bug.cgi?id=2079311
>
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
> arch/x86/kvm/cpuid.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index de6d44e07e34..3b2319cecfd1 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -298,7 +298,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> guest_supported_xcr0 =
> cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);
>
> - vcpu->arch.guest_fpu.fpstate->user_xfeatures = guest_supported_xcr0;
> + vcpu->arch.guest_fpu.fpstate->user_xfeatures = guest_supported_xcr0 |
> + XFEATURE_MASK_FPSSE;

I don't think this is correct. This will allow the guest to set the SSE bit
even when XSAVE isn't supported due to kvm_guest_supported_xcr0() returning
user_xfeatures.

static inline u64 kvm_guest_supported_xcr0(struct kvm_vcpu *vcpu)
{
return vcpu->arch.guest_fpu.fpstate->user_xfeatures;
}

I believe the right place to fix this is in validate_user_xstate_header(). It's
reachable if and only if XSAVE is supported in the host, and when XSAVE is _not_
supported, the kernel unconditionally allows FP+SSE. So it follows that the kernel
should also allow FP+SSE when using XSAVE too. That would also align the logic
with fpu_copy_guest_fpstate_to_uabi(), which fordces the FPSSE flags. Ditto for
the non-KVM save_xstate_epilog().

Aha! And fpu__init_system_xstate() ensure the host supports FP+SSE when XSAVE
is enabled (knew their had to be a sanity check somewhere).

---
arch/x86/kernel/fpu/xstate.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index c8340156bfd2..83b9a9653d47 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -399,8 +399,13 @@ int xfeature_size(int xfeature_nr)
static int validate_user_xstate_header(const struct xstate_header *hdr,
struct fpstate *fpstate)
{
- /* No unknown or supervisor features may be set */
- if (hdr->xfeatures & ~fpstate->user_xfeatures)
+ /*
+ * No unknown or supervisor features may be set. Userspace is always
+ * allowed to restore FP+SSE state (XSAVE/XRSTOR are used by the kernel
+ * if and only if FP+SSE are supported in xstate).
+ */
+ if (hdr->xfeatures & ~fpstate->user_xfeatures &
+ ~(XFEATURE_MASK_FP | XFEATURE_MASK_SSE))
return -EINVAL;

/* Userspace must use the uncompacted format */
base-commit: de3d415edca23831c5d1f24f10c74a715af7efdb
--

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