On Tue, Dec 30, 2025, Paolo Bonzini wrote:
On Mon, Dec 29, 2025 at 11:45 PM Sean Christopherson seanjc@google.com wrote:
So, given that KVM's effective ABI is to record XSTATE_BV[i]=0 if XFD[i]==1, I vote to fix this by emulating that behavior when stuffing XFD in fpu_update_guest_xfd(), and then manually closing the hole Paolo found in fpu_copy_uabi_to_guest_fpstate().
I disagree with changing the argument from const void* to void*. Let's instead treat it as a KVM backwards-compatibility quirk:
union fpregs_state *xstate = (union fpregs_state *)guest_xsave->region; xstate->xsave.header.xfeatures &= ~vcpu->arch.guest_fpu.fpstate->xfd;It keeps the kernel/ API const as expected and if anything I'd consider adding a WARN to fpu_copy_uabi_to_guest_fpstate(), basically asserting that there would be no #NM on the subsequent restore.
Works for me.
@@ -319,10 +319,25 @@ EXPORT_SYMBOL_FOR_KVM(fpu_enable_guest_xfd_features); #ifdef CONFIG_X86_64 void fpu_update_guest_xfd(struct fpu_guest *guest_fpu, u64 xfd) {
struct fpstate *fpstate = guest_fpu->fpstate;fpregs_lock();
guest_fpu->fpstate->xfd = xfd;if (guest_fpu->fpstate->in_use)xfd_update_state(guest_fpu->fpstate);
fpstate->xfd = xfd;if (fpstate->in_use)xfd_update_state(fpstate);/** If the guest's FPU state is NOT resident in hardware, clear disabled* components in XSTATE_BV as attempting to load disabled components* will generate #NM _in the host_, and KVM's ABI is that saving guest* XSAVE state should see XSTATE_BV[i]=0 if XFD[i]=1.** If the guest's FPU state is in hardware, simply do nothing as XSAVE* itself saves XSTATE_BV[i] as 0 if XFD[i]=1.s/saves/(from fpu_swap_kvm_fpstate) will save/
*/if (xfd && test_thread_flag(TIF_NEED_FPU_LOAD))fpstate->regs.xsave.header.xfeatures &= ~xfd;No objections to this part. I'll play with this to adjust the selftests either tomorrow or, more likely, on January 2nd, and send a v2 that also includes the change from preemption_disabled to irqs_disabled.
To hopefully save you some time, I responded to the selftests with cleanups and adjustments to hit both bugs (see patch 3).
I take it that you don't have any qualms with the new fpu_load_guest_fpstate function,
Hmm, I don't have a strong opinion? Actually, after looking at patch 5, I agree that adding fpu_load_guest_fpstate() is useful. My only hesitation was that kvm_fpu_{get,put}() would be _very_ similar, but critically different, at which point NOT using fpu_update_guest_xfd() in kvm_fpu_get() could be confusing.
but let me know if you prefer to have it in a separate submission destined to 6.20 only.
I'd say don't send it to stable@, otherwise I don't have a preference on 6.19 versus 6.20.