linux-kernel - Re: [RFC PATCH v2 14/23] KVM: TDX: Split and inhibit huge mappings if a VMExit carries level info

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Message-ID: <aRaJE6s8AihGfh8w@yzhao56-desk.sh.intel.com>
Date: Fri, 14 Nov 2025 09:42:43 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>
CC: "pbonzini@...hat.com" <pbonzini@...hat.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "quic_eberman@...cinc.com" <quic_eberman@...cinc.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Li, Xiaoyao"
	<xiaoyao.li@...el.com>, "Du, Fan" <fan.du@...el.com>, "Hansen, Dave"
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Subject: Re: [RFC PATCH v2 14/23] KVM: TDX: Split and inhibit huge mappings
 if a VMExit carries level info

On Tue, Nov 11, 2025 at 06:55:45PM +0800, Huang, Kai wrote:
> On Thu, 2025-08-07 at 17:44 +0800, Yan Zhao wrote:
> > @@ -2044,6 +2091,9 @@ static int tdx_handle_ept_violation(struct kvm_vcpu *vcpu)
> >  		 */
> >  		exit_qual = EPT_VIOLATION_ACC_WRITE;
> >  
> > +		if (tdx_check_accept_level(vcpu, gpa_to_gfn(gpa)))
> > +			return RET_PF_RETRY;
> > +
> 
> I don't think you should return RET_PF_RETRY here.
> 
> This is still at very early stage of EPT violation.  The caller of
> tdx_handle_ept_violation() is expecting either 0, 1, or negative error code.
Hmm, strictly speaking, the caller of the EPT violation handler is expecting
0, >0, or negative error code.

vcpu_run
  |->r = vcpu_enter_guest(vcpu);
  |        |->r = kvm_x86_call(handle_exit)(vcpu, exit_fastpath);
  |        |  return r;
  |  if (r <= 0)
  |     break;

handle_ept_violation
  |->return __vmx_handle_ept_violation(vcpu, gpa, exit_qualification);

tdx_handle_ept_violation
 |->ret = __vmx_handle_ept_violation(vcpu, gpa, exit_qual); 
 |  return ret;

The current VMX/TDX's EPT violation handlers returns RET_PF_* to the caller
since commit 7c5480386300 ("KVM: x86/mmu: Return RET_PF* instead of 1 in
kvm_mmu_page_fault") for the sake of zero-step mitigation.

This is no problem, because

enum {
        RET_PF_CONTINUE = 0,
        RET_PF_RETRY,
        RET_PF_EMULATE,
        RET_PF_WRITE_PROTECTED,
        RET_PF_INVALID,
        RET_PF_FIXED,
        RET_PF_SPURIOUS,
};

/*
 * Define RET_PF_CONTINUE as 0 to allow for
 * - efficient machine code when checking for CONTINUE, e.g.
 *   "TEST %rax, %rax, JNZ", as all "stop!" values are non-zero,
 * - kvm_mmu_do_page_fault() to return other RET_PF_* as a positive value.
 */
static_assert(RET_PF_CONTINUE == 0);