Re: [Xen-devel] [PATCH 1/3] x86/emul: Optimise decode_register() somewhat

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.01.18 at 14:16, <andrew.cooper3@xxxxxxxxxx> wrote:
> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> @@ -396,6 +396,51 @@ static const struct {
>  /* Shift values between src and dst sizes of pmov{s,z}x{b,w,d}{w,d,q}. */
>  static const uint8_t pmov_convert_delta[] = { 1, 2, 3, 1, 2, 1 };
>  
> +/*
> + * Map GPRs by ModRM encoding to their offset within struct cpu_user_regs.
> + * The AH,CH,DH,BH offsets are misaligned.
> + */
> +static const uint8_t cpu_user_regs_gpr_offsets[] = {
> +    offsetof(struct cpu_user_regs, r(ax)),
> +    offsetof(struct cpu_user_regs, r(cx)),
> +    offsetof(struct cpu_user_regs, r(dx)),
> +    offsetof(struct cpu_user_regs, r(bx)),
> +    offsetof(struct cpu_user_regs, r(sp)),
> +    offsetof(struct cpu_user_regs, r(bp)),
> +    offsetof(struct cpu_user_regs, r(si)),
> +    offsetof(struct cpu_user_regs, r(di)),
> +#if defined(__x86_64__)

#ifdef ?

> +    offsetof(struct cpu_user_regs, r8),
> +    offsetof(struct cpu_user_regs, r9),
> +    offsetof(struct cpu_user_regs, r10),
> +    offsetof(struct cpu_user_regs, r11),
> +    offsetof(struct cpu_user_regs, r12),
> +    offsetof(struct cpu_user_regs, r13),
> +    offsetof(struct cpu_user_regs, r14),
> +    offsetof(struct cpu_user_regs, r15),
> +#endif
> +};
> +static const uint8_t cpu_user_regs_high_gpr_offsets[] = {
> +    offsetof(struct cpu_user_regs, r(ax)),
> +    offsetof(struct cpu_user_regs, r(cx)),
> +    offsetof(struct cpu_user_regs, r(dx)),
> +    offsetof(struct cpu_user_regs, r(bx)),

al, cl, dl, bl respectively?

> +    offsetof(struct cpu_user_regs, ah),
> +    offsetof(struct cpu_user_regs, ch),
> +    offsetof(struct cpu_user_regs, dh),
> +    offsetof(struct cpu_user_regs, bh),
> +#if defined(__x86_64__)
> +    offsetof(struct cpu_user_regs, r8),
> +    offsetof(struct cpu_user_regs, r9),
> +    offsetof(struct cpu_user_regs, r10),
> +    offsetof(struct cpu_user_regs, r11),
> +    offsetof(struct cpu_user_regs, r12),
> +    offsetof(struct cpu_user_regs, r13),
> +    offsetof(struct cpu_user_regs, r14),
> +    offsetof(struct cpu_user_regs, r15),
> +#endif

These 8 entries are pointless - there's no encoding which could
result in them being used.

Not also that the name of the array isn't reflecting its contents: Only
four of the entries actually are "high byte" registers.

I also think both arrays would better have function scope only.

Jan


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