Re: [PATCH v6 01/20] KVM: x86: Extend VCPU registers for EGPRs

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] The patch extends the VCPU register array (`vcpu->arch.regs`) for APX Extended General-Purpose Registers (R16-R31) but omits the required synchronization logic with the guest hardware or XSAVE state, leading to broken register caching and silent guest state corruption.
--

--- Patch [1]: [PATCH v6 01/20] KVM: x86: Extend VCPU registers for EGPRs ---
commit c513074e3dcbca9bda2fa54321ecd4e8c1d2229b
Author: Chang S. Bae <chang.seok.bae@intel.com>

KVM: x86: Extend VCPU registers for EGPRs

This commit extends the VCPU register storage array to include APX Extended
General-Purpose Registers (EGPRs R16-R31) for VMX when KVM_APX is enabled.
It uses VCPU storage to provide an access path for KVM instead of directly
reading the guest fpstate.

Link: https://lore.kernel.org/abL8SW5JS1aV5goa@google.com

> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index b517257a63153..48547842d9fa0 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
[ ... ]
> @@ -178,6 +178,24 @@ enum kvm_reg {
>  	VCPU_REGS_R13,
>  	VCPU_REGS_R14,
>  	VCPU_REGS_R15,
> +#endif
> +#ifdef CONFIG_KVM_APX
> +	VCPU_REGS_R16,
> +	VCPU_REGS_R17,

[Severity: High]
Does adding these registers before NR_VCPU_GENERAL_PURPOSE_REGS break KVM's
register caching assumptions?

KVM assumes all registers defined before NR_VCPU_GENERAL_PURPOSE_REGS are
either eagerly saved and restored by assembly context switches or explicitly
defined in the lazy load set. Because R16-R31 are not in the lazy load set,
KVM's initialization will set their availability bits to 1, assuming they are
always synchronized by hardware context switches.

However, the context switch assembly only saves and restores up to R15 and
does not touch R16-R31. When the KVM host emulator accesses R16-R31 to
emulate an instruction, won't it read stale or uninitialized data from the
software array? 

Furthermore, if KVM writes to these registers during emulation, won't those
writes be silently lost upon resuming the guest since they are never
synchronized back to the guest's XSAVE state?

> +	VCPU_REGS_R18,
> +	VCPU_REGS_R19,
[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260729191656.598771-1-chang.seok.bae@intel.com?part=1