Re: [PATCH v3 03/10] KVM: selftests: Use an array for guest_regs (and fix offsets)

[Yosry Ahmed]

On Fri, Jul 24, 2026 at 10:03 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Fri, Jul 24, 2026, Yosry Ahmed wrote:
> > On Fri, Jul 24, 2026 at 8:33 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > Oh I wanted to, but the compiler wouldn't let me use enum values
> > directly in the preprocessor, see:
> > https://lore.kernel.org/kvm/CAO9r8zPz8WbijVN6mgLb8OaU6=tNSSz1NgNBjZCsC36D4+TO+w@xxxxxxxxxxxxxx/.
> >
> > I can do an enum *and* macros using enum values if you prefer so, like:
> >
> > #define __GUEST_REGS_RAX 0
> > ...
> >
> > enum guest_regs {
> >     GUEST_REGS_RAX = __GUEST_REGS_RAX,
> >     ...
> >     NR_GUEST_REGS,
> > };
>
> I would much rather follow KUT's lib/x86/virt.h, and provide asm constraints for
> the offsets.

I actually did this initially (without knowing it's what KUT does), I
did not like the asm constraint, and I think I may have run into a
limit on the number of operands. But yeah if we are already doing this
for KUT let me try it again and see if I can make it work.

> Actually, my vote is to copy+paste KUT's entire approach, and provide:
>
>   struct guest_regs {
>         u64 rax;
>         u64 rcx;
>         u64 rdx;
>         u64 rbx;
>         /*
>          * Use RSP's index to hold CR2, as RSP isn't manually context switched
>          * by software in any relevant flows.
>          *
>          * FIXME: Actually context switch CR2 on Intel.
>          */
>         u64 cr2;
>         u64 rbp;
>         u64 rsi;
>         u64 rdi;
>         u64 r8;
>         u64 r9;
>         u64 r10;
>         u64 r11;
>         u64 r12;
>         u64 r13;
>         u64 r14;
>         u64 r15;
>         u64 rflags;
>   };
>
> Using an array for the GPRs is only valuable when the code *needs* to index the
> GPRs based on their index, e.g. like KVM needs to do when emulating instructions.
> But for "normal" software, using an array just makes the code kludgy.
>
> E.g. this
>
>         vmcb->save.rip = (u64)guest_rip;
>         vmcb->save.rsp = (u64)svm->stack;
>         guest_regs.rdi = (u64)svm;
>
> versus
>
>         vmcb->save.rip = (u64)guest_rip;
>         vmcb->save.rsp = (u64)svm->stack;
>         guest_regs[X86_REGS_RDI] = (u64)svm;