Re: [PATCH v2 01/16] KVM: x86: Rename register accessors to be GPR-specific

[Sean Christopherson <seanjc@google.com>]

On Tue, Mar 10, 2026, Chang S. Bae wrote:
> On 3/9/2026 6:23 PM, Sean Christopherson wrote:
> > 
> > Oh, yikes, I didn't even see that this series is playing games with the register
> > indices.
> > 
> > Whatever we do, the changelog asbolutely needs to call out the real motiviation.
> 
> Given the discussion here, it looks so apparent the changelog is missing
> that detail. I'll ensure something like what you wrote here to the revision.
> 
> > I'll try to come back to this tomorrow with more complete thoughts and hopefully
> 
> Sure, you call it. I know you have a lot on your plate, so I hope you feel
> free to take your time. Thanks!
> 
> > E.g. passing in VCPU_REGS_RIP to kvm_gpr_read() will compile just fine, but will
> > read the wrong register on APX capable hardware.
> 
> Right, so new semantics likely need to be established. As responded before,
> one option would be separate them in structure:
> 
> diff --git a/arch/x86/include/asm/kvm_host.h
> b/arch/x86/include/asm/kvm_host.h
> index ff07c45e3c73..ff8a317be5cf 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -795,10 +795,14 @@ enum kvm_only_cpuid_leafs {
> 
>  struct kvm_vcpu_arch {
>         /*
> -        * rip and regs accesses must go through
> -        * kvm_{register,rip}_{read,write} functions.
> +        * regs accesses must go through kvm_register_{read,write}
> +        * functions.
>          */
>         unsigned long regs[NR_VCPU_REGS];
> +
> +       /* rip accesses must go through kvm_rip_{read,write} */
> +       unsigned long rip;

Ya, this is where I ended up too.  And then as prep work, we can and should
convert regs_{avail,dirty} to proper bitmaps so that the size can be dynamic
for 32-bit vs. 64-bit vs. APX-capable (or we could just use a "unsigned long",
it would only change what BUILD_BUG_ON()s are needed).

E.g. I have

        unsigned long regs[NR_VCPU_GENERAL_PURPOSE_REGS];
        unsigned long rip;
        DECLARE_BITMAP(regs_avail, NR_VCPU_TOTAL_REGS);
        DECLARE_BITMAP(regs_dirty, NR_VCPU_TOTAL_REGS);

and then the below as a final testing hack for APX.  I should be able to post a
small series later today, which will map out out most of the register crud (I
didn't do anything with the emulator, so it's not a complete prep series, but
it should be enough to allow us to choose a direction).

enum kvm_reg {
        VCPU_REGS_RAX = __VCPU_REGS_RAX,
        VCPU_REGS_RCX = __VCPU_REGS_RCX,
        VCPU_REGS_RDX = __VCPU_REGS_RDX,
        VCPU_REGS_RBX = __VCPU_REGS_RBX,
        VCPU_REGS_RSP = __VCPU_REGS_RSP,
        VCPU_REGS_RBP = __VCPU_REGS_RBP,
        VCPU_REGS_RSI = __VCPU_REGS_RSI,
        VCPU_REGS_RDI = __VCPU_REGS_RDI,
#ifdef CONFIG_X86_64
        VCPU_REGS_R8  = __VCPU_REGS_R8,
        VCPU_REGS_R9  = __VCPU_REGS_R9,
        VCPU_REGS_R10 = __VCPU_REGS_R10,
        VCPU_REGS_R11 = __VCPU_REGS_R11,
        VCPU_REGS_R12 = __VCPU_REGS_R12,
        VCPU_REGS_R13 = __VCPU_REGS_R13,
        VCPU_REGS_R14 = __VCPU_REGS_R14,
        VCPU_REGS_R15 = __VCPU_REGS_R15,
#define CONFIG_X86_APX

#endif

#ifdef CONFIG_X86_APX
        VCPU_REG_R16 = VCPU_REGS_R15 + 1,
        VCPU_REG_R17,
        VCPU_REG_R18,
        VCPU_REG_R19,
        VCPU_REG_R20,
        VCPU_REG_R21,
        VCPU_REG_R22,
        VCPU_REG_R23,
        VCPU_REG_R24,
        VCPU_REG_R25,
        VCPU_REG_R26,
        VCPU_REG_R27,
        VCPU_REG_R28,
        VCPU_REG_R29,
        VCPU_REG_R30,
        VCPU_REG_R31,
#endif
        NR_VCPU_GENERAL_PURPOSE_REGS,

        VCPU_REG_RIP = NR_VCPU_GENERAL_PURPOSE_REGS,

        VCPU_REG_PDPTR,
        VCPU_REG_CR0,
        /*
         * Alias AMD's ERAPS (not a real register) to CR3 so that common code
         * can trigger emulation of the RAP (Return Address Predictor) with
         * minimal support required in common code.  Piggyback CR3 as the RAP
         * is cleared on writes to CR3, i.e. marking CR3 dirty will naturally
         * mark ERAPS dirty as well.
         */
        VCPU_REG_CR3,
        VCPU_REG_ERAPS = VCPU_REG_CR3,
        VCPU_REG_CR4,
        VCPU_REG_RFLAGS,
        VCPU_REG_SEGMENTS,
        VCPU_REG_EXIT_INFO_1,
        VCPU_REG_EXIT_INFO_2,

        NR_VCPU_TOTAL_REGS,
};