Re: [PATCH 04/11] KVM: VMX: Read 32-bit GPR values for ENCLS instructions outside of 64-bit mode

[Sean Christopherson <seanjc@google.com>]

On Mon, Apr 13, 2026, Kai Huang wrote:
> On Thu, 2026-04-09 at 16:56 -0700, Sean Christopherson wrote:
> > When getting register values for ENCLS emulation, use kvm_register_read()
> > instead of kvm_<reg>_read() so that bits 63:32 of the register are dropped
> > if the guest is in 32-bit mode.
> > 
> > Note, the misleading/surprising behavior of kvm_<reg>_read() being "raw"
> > variants under the hood will be addressed once all non-benign bugs are
> > fixed.
> > 
> > Fixes: 70210c044b4e ("KVM: VMX: Add SGX ENCLS[ECREATE] handler to enforce CPUID restrictions")
> > Fixes: b6f084ca5538 ("KVM: VMX: Add ENCLS[EINIT] handler to support SGX Launch Control (LC)")
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  arch/x86/kvm/vmx/sgx.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/vmx/sgx.c b/arch/x86/kvm/vmx/sgx.c
> > index df1d0cf76947..4c61fc33f764 100644
> > --- a/arch/x86/kvm/vmx/sgx.c
> > +++ b/arch/x86/kvm/vmx/sgx.c
> > @@ -225,8 +225,8 @@ static int handle_encls_ecreate(struct kvm_vcpu *vcpu)
> >  	struct x86_exception ex;
> >  	int r;
> >  
> > -	if (sgx_get_encls_gva(vcpu, kvm_rbx_read(vcpu), 32, 32, &pageinfo_gva) ||
> > -	    sgx_get_encls_gva(vcpu, kvm_rcx_read(vcpu), 4096, 4096, &secs_gva))
> > +	if (sgx_get_encls_gva(vcpu, kvm_register_read(vcpu, VCPU_REGS_RBX), 32, 32, &pageinfo_gva) ||
> > +	    sgx_get_encls_gva(vcpu, kvm_register_read(vcpu, VCPU_REGS_RCX), 4096, 4096, &secs_gva))
> >  		return 1;
> >  
> >  	/*
> > @@ -302,9 +302,9 @@ static int handle_encls_einit(struct kvm_vcpu *vcpu)
> >  	gpa_t sig_gpa, secs_gpa, token_gpa;
> >  	int ret, trapnr;
> >  
> > -	if (sgx_get_encls_gva(vcpu, kvm_rbx_read(vcpu), 1808, 4096, &sig_gva) ||
> > -	    sgx_get_encls_gva(vcpu, kvm_rcx_read(vcpu), 4096, 4096, &secs_gva) ||
> > -	    sgx_get_encls_gva(vcpu, kvm_rdx_read(vcpu), 304, 512, &token_gva))
> > +	if (sgx_get_encls_gva(vcpu, kvm_register_read(vcpu, VCPU_REGS_RBX), 1808, 4096, &sig_gva) ||
> > +	    sgx_get_encls_gva(vcpu, kvm_register_read(vcpu, VCPU_REGS_RCX), 4096, 4096, &secs_gva) ||
> > +	    sgx_get_encls_gva(vcpu, kvm_register_read(vcpu, VCPU_REGS_RDX), 304, 512, &token_gva))
> >  		return 1;
> > 
> 
> Is there any case where bits 63:32 can have non-zero value?

Yes, GPR values aren't modified on transitions to/from 64-bit mode.  E.g. if
software loads 64-bit values in 64-bit mode, under the hood those values will
still be there while the CPU is in 32-bit/compat mode.

Well, that's not strictly true, per the SDM.  Values are only preseverd for R8-R15
on compat<=>64-bit transitions:

  Registers only available in 64-bit mode (R8-R15 and XMM8-XMM15) are preserved
  across transitions from 64-bit mode into compatibility mode then back into
  64-bit mode. However, values of R8-R15 and XMM8-XMM15 are unde- fined after
  transitions from 64-bit mode through compatibility mode to legacy or real mode
  and then back through compatibility mode to 64-bit mode.

And "legacy" GPRs are never preserved:

  Because the upper 32 bits of 64-bit general-purpose registers are undefined in
  32-bit modes, the upper 32 bits of any general-purpose register are not preserved
  when switching from 64-bit mode to a 32-bit mode (to protected mode or
  compatibility mode). Software must not depend on these bits to maintain a value
  after a 64-bit to 32-bit mode switch.

But IIRC, that's "just" the architectural behavior.  Hardware implementations may
choose to preserve values.

> If vCPU is in 32-bit mode then it should not be able to access 64-bit GPR?

Yes and no.  Mostly no.  Architecturally, they're all off limits.  But, again
going from memory that's ~15 years old at this point, IIRC the behavior is that
writes in 32-bit modes zero bits 63:32, same as 32-bit writes in 64-bit mode.

Take all of my memory with a huge grain of salt, it's very possible I'm
mis-remembering hallway discussions from a long time ago.