Re: [PATCH 2/2] KVM: x86/mmu: Bail out kvm_tdp_map_page() when VM dead

[Yan Zhao <yan.y.zhao@intel.com>]

On Tue, Feb 18, 2025 at 08:03:57AM -0800, Sean Christopherson wrote:
> On Mon, Feb 17, 2025, Yan Zhao wrote:
> > Bail out of the loop in kvm_tdp_map_page() when a VM is dead. Otherwise,
> > kvm_tdp_map_page() may get stuck in the kernel loop when there's only one
> > vCPU in the VM (or if the other vCPUs are not executing ioctls), even if
> > fatal errors have occurred.
> > 
> > kvm_tdp_map_page() is called by the ioctl KVM_PRE_FAULT_MEMORY or the TDX
> > ioctl KVM_TDX_INIT_MEM_REGION. It loops in the kernel whenever RET_PF_RETRY
> > is returned. In the TDP MMU, kvm_tdp_mmu_map() always returns RET_PF_RETRY,
> > regardless of the specific error code from tdp_mmu_set_spte_atomic(),
> > tdp_mmu_link_sp(), or tdp_mmu_split_huge_page(). While this is acceptable
> > in general cases where the only possible error code from these functions is
> > -EBUSY, TDX introduces an additional error code, -EIO, due to SEAMCALL
> > errors.
> > 
> > Since this -EIO error is also a fatal error, check for VM dead in the
> > kvm_tdp_map_page() to avoid unnecessary retries until a signal is pending.
> > 
> > The error -EIO is uncommon and has not been observed in real workloads.
> > Currently, it is only hypothetically triggered by bypassing the real
> > SEAMCALL and faking an error in the SEAMCALL wrapper.
> > 
> > Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
> > ---
> >  arch/x86/kvm/mmu/mmu.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 08ed5092c15a..3a8d735939b5 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -4700,6 +4700,10 @@ int kvm_tdp_map_page(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code, u8 *level
> >  	do {
> >  		if (signal_pending(current))
> >  			return -EINTR;
> > +
> > +		if (vcpu->kvm->vm_dead)
> 
> This needs to be READ_ONCE().  Along those lines, I think I'd prefer
Indeed.

> 
> 		if (kvm_check_request(KVM_REQ_VM_DEAD, vcpu))
> 			return -EIO;
> 
> or
> 
> 		if (kvm_check_request(KVM_REQ_VM_DEAD, vcpu)) 
> 			return -EIO;
Hmm, what's the difference between the two cases?
Paste error?

> so that if more terminal requests come long, we can bundle everything into a
> single check via a selective version of kvm_request_pending().
Makes sense!
I'll update it to
 		if (kvm_check_request(KVM_REQ_VM_DEAD, vcpu)) 
 			return -EIO;
in v2.