Re: [PATCH v15 07/23] KVM: selftests: Add TDX boot code

[Lisa Wang <wyihan@google.com>]

On Fri, Oct 02, 2026 at 09:13:29AM +0000, sashiko-bot@kernel.org wrote:
> [ ... ]
> 
> > +	/*
> > +	 * Find the address of struct td_per_vcpu_parameters for this
> > +	 * vCPU based on esi (TDX spec: initialized with vCPU id). Put
> > +	 * struct address into eax for indirect addressing.
> > +	 */
> > +	movl $SIZEOF_TD_PER_VCPU_PARAMETERS, %eax
> > +	mul %esi
> 
> [Severity: Medium]
> Is it correct to read the vCPU index from %esi in td_boot here?
> 
> According to the TDX architecture specification (via TDH.VP.INIT), the vCPU
> index is initialized in the %ecx register, while %esi is initialized to 0.
> 
> If %esi is used, will all vCPUs evaluate to index 0, causing them to
> incorrectly share the same stack and guest code pointer? This seems like it
> could corrupt the SMP stack and fail multi-vCPU tests.

According to the TDX spec, %esi is the vCPU index.

> [ ... ]
> 
> > +	/*
> > +	 * Set up control registers (x86 does not support moving memory
> > +	 * directly to a control register).
> > +	 */
> > +	movl TD_BOOT_PARAMETERS_CR4(%ebx), %edi
> > +	movl %edi, %cr4
> > +	movl TD_BOOT_PARAMETERS_CR3(%ebx), %edi
> > +	movl %edi, %cr3
> > +	movl TD_BOOT_PARAMETERS_CR0(%ebx), %edi
> > +	movl %edi, %cr0
> > +
> > +	/* Switching to 64bit mode after ljmp and then jump to guest code */
> > +	ljmp $(KERNEL_CS),$1f
> 
> [Severity: Medium]
> Does td_boot miss a wrmsr to set EFER.LME before enabling paging?
> 
> The TDX VM guest boot code enables paging (CR0.PG) and physical address
> extensions (CR4.PAE) here, but appears to omit writing the EFER MSR to set
> the Long Mode Enable (LME) bit.
> 
> According to the x86 architecture, enabling paging without setting EFER.LME
> puts the processor into 32-bit PAE paging mode instead of 64-bit IA-32e
> mode.
> 
> Will the subsequent far jump attempt to load a 64-bit code segment
> (KERNEL_CS has L=1), causing a #GP fault since it is invalid in 32-bit mode?
> It seems this might crash the guest VM unconditionally on every boot.

In TDX, TDH.VP.INIT initializes EFER.LME = 1 already.

> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20261001-tdx-selftests-v15-0-7c62a5d8a992@google.com?part=7