Re: [patch 16/31] x86/fpu: Replace KVMs homebrewn FPU copy to user

Thomas Gleixner
2021-10-12T17:47:18Z

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