linux-kernel - Re: [PATCH 2/2] KVM: x86/mmu: Bail out kvm_tdp_map

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Message-ID: <Z7XoQU-kEF8osICK@google.com>
Date: Wed, 19 Feb 2025 06:18:41 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: pbonzini@...hat.com, rick.p.edgecombe@...el.com, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: x86/mmu: Bail out kvm_tdp_map_page() when VM dead

On Wed, Feb 19, 2025, Yan Zhao wrote:
> 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@...el.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?

Hrm, yes.  I already forgot what I was thinking, but I believe the second one was
supposed to be:

		if (kvm_test_request(KVM_REQ_VM_DEAD, vcpu))
			return -EIO;

The "check" version should be fine though, i.e. clearing the request is ok,
because kvm_vcpu_ioctl() will see vcpu->kvm->vm_dead before handling KVM_RUN or
any other ioctl.