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    SubjectRe: [patch 16/31] x86/fpu: Replace KVMs homebrewn FPU copy to user
    Date
    On Tue, Oct 12 2021 at 19:36, Paolo Bonzini wrote:
    > On 12/10/21 02:00, Thomas Gleixner wrote:
    >>
    >> - if (boot_cpu_has(X86_FEATURE_XSAVE)) {
    >> - memset(guest_xsave, 0, sizeof(struct kvm_xsave));
    >> - fill_xsave((u8 *) guest_xsave->region, vcpu);
    >> - } else {
    >> - memcpy(guest_xsave->region,
    >> - &vcpu->arch.guest_fpu->state.fxsave,
    >> - sizeof(struct fxregs_state));
    >> - *(u64 *)&guest_xsave->region[XSAVE_HDR_OFFSET / sizeof(u32)] =
    >> - XFEATURE_MASK_FPSSE;
    >> - }
    >
    > After the patch, this final assignment is not done in the else case:

    Doh.

    >> +
    >> + if (cpu_feature_enabled(X86_FEATURE_XSAVE)) {
    >> + __copy_xstate_to_uabi_buf(mb, &kstate->xsave, pkru,
    >> + XSTATE_COPY_XSAVE);
    >> + } else {
    >> + memcpy(&ustate->fxsave, &kstate->fxsave, sizeof(ustate->fxsave));
    >> + }
    >> +}
    >
    > This leaves the xstate_bv set to 0 instead of XFEATURE_MASK_FPSSE.
    > Resuming a VM then fails if you save on a non-XSAVE machine and restore
    > it on an XSAVE machine.

    Yup.

    > The memset(guest_xsave, 0, sizeof(struct kvm_xsave)) also is not
    > reproduced, you can make it unconditional for simplicity; this is not a
    > fast path.

    Duh, I should have mentioned that in the changelog. The buffer is
    allocated with kzalloc() soe the memset is redundant, right?

    Thanks,

    tglx

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