Re: [PATCH] x86/domctl: Reorder user_regs accesses for efficiency

On Mon, 20 Jul 2026 at 10:38, Andrew Cooper <[email protected]> wrote:
>
> The current order is derived from expanding XLAT_cpu_user_regs() which uses
> the x86_32 cpu_user_regs order.  This is correct for one the blocks, but not
> the others.
>
> In order to work with the prefetcher, rather than against it, loads want to be
> in field order.  Make this so for the three paths reading an x86_64
> cpu_user_regs.
>
> No functional change.
>
> Reported-by: Jan Beulich <[email protected]>
> Signed-off-by: Andrew Cooper <[email protected]>
> ---
> CC: Jan Beulich <[email protected]>
> CC: Roger Pau Monné <[email protected]>
> CC: Teddy Astie <[email protected]>
> CC: Marek Marczykowski-Górecki <[email protected]>
> ---
>  xen/arch/x86/domain.c |  6 +++---
>  xen/arch/x86/domctl.c | 10 +++++-----
>  2 files changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
> index 53a86ba373a4..996b50af7a17 100644
> --- a/xen/arch/x86/domain.c
> +++ b/xen/arch/x86/domain.c
> @@ -1259,17 +1259,17 @@ int arch_set_info_guest(
>          v->arch.user_regs.r14               = c.nat->user_regs.r14;
>          v->arch.user_regs.r13               = c.nat->user_regs.r13;
>          v->arch.user_regs.r12               = c.nat->user_regs.r12;
> +        v->arch.user_regs.rbp               = c.nat->user_regs.rbp;
> +        v->arch.user_regs.rbx               = c.nat->user_regs.rbx;
>          v->arch.user_regs.r11               = c.nat->user_regs.r11;
>          v->arch.user_regs.r10               = c.nat->user_regs.r10;
>          v->arch.user_regs.r9                = c.nat->user_regs.r9;
>          v->arch.user_regs.r8                = c.nat->user_regs.r8;
> -        v->arch.user_regs.rbx               = c.nat->user_regs.rbx;
> +        v->arch.user_regs.rax               = c.nat->user_regs.rax;
>          v->arch.user_regs.rcx               = c.nat->user_regs.rcx;
>          v->arch.user_regs.rdx               = c.nat->user_regs.rdx;
>          v->arch.user_regs.rsi               = c.nat->user_regs.rsi;
>          v->arch.user_regs.rdi               = c.nat->user_regs.rdi;
> -        v->arch.user_regs.rbp               = c.nat->user_regs.rbp;
> -        v->arch.user_regs.rax               = c.nat->user_regs.rax;
>          v->arch.user_regs.rip               = c.nat->user_regs.rip;
>          v->arch.user_regs.cs                = c.nat->user_regs.cs;
>          v->arch.user_regs.rflags            = (c.nat->user_regs.rflags & X86_EFLAGS_ALL) | X86_EFLAGS_MBS;
> diff --git a/xen/arch/x86/domctl.c b/xen/arch/x86/domctl.c
> index 3ea4c650eee0..2ecd81d89b09 100644
> --- a/xen/arch/x86/domctl.c
> +++ b/xen/arch/x86/domctl.c
> @@ -1485,17 +1485,17 @@ void arch_get_info_guest(struct vcpu *v, vcpu_guest_context_u c)
>          c.nat->user_regs.r14               = v->arch.user_regs.r14;
>          c.nat->user_regs.r13               = v->arch.user_regs.r13;
>          c.nat->user_regs.r12               = v->arch.user_regs.r12;
> +        c.nat->user_regs.rbp               = v->arch.user_regs.rbp;
> +        c.nat->user_regs.rbx               = v->arch.user_regs.rbx;
>          c.nat->user_regs.r11               = v->arch.user_regs.r11;
>          c.nat->user_regs.r10               = v->arch.user_regs.r10;
>          c.nat->user_regs.r9                = v->arch.user_regs.r9;
>          c.nat->user_regs.r8                = v->arch.user_regs.r8;
> -        c.nat->user_regs.rbx               = v->arch.user_regs.rbx;
> +        c.nat->user_regs.rax               = v->arch.user_regs.rax;
>          c.nat->user_regs.rcx               = v->arch.user_regs.rcx;
>          c.nat->user_regs.rdx               = v->arch.user_regs.rdx;
>          c.nat->user_regs.rsi               = v->arch.user_regs.rsi;
>          c.nat->user_regs.rdi               = v->arch.user_regs.rdi;
> -        c.nat->user_regs.rbp               = v->arch.user_regs.rbp;
> -        c.nat->user_regs.rax               = v->arch.user_regs.rax;
>          c.nat->user_regs.rip               = v->arch.user_regs.rip;
>          c.nat->user_regs.cs                = v->arch.user_regs.cs;
>          c.nat->user_regs.rflags            = v->arch.user_regs.rflags;
> @@ -1514,13 +1514,13 @@ void arch_get_info_guest(struct vcpu *v, vcpu_guest_context_u c)
>      else
>      {
>          /* Backing memory is pre-zeroed. */
> +        c.cmp->user_regs.ebp               = v->arch.user_regs.ebp;
>          c.cmp->user_regs.ebx               = v->arch.user_regs.ebx;
> +        c.cmp->user_regs.eax               = v->arch.user_regs.eax;
>          c.cmp->user_regs.ecx               = v->arch.user_regs.ecx;
>          c.cmp->user_regs.edx               = v->arch.user_regs.edx;
>          c.cmp->user_regs.esi               = v->arch.user_regs.esi;
>          c.cmp->user_regs.edi               = v->arch.user_regs.edi;
> -        c.cmp->user_regs.ebp               = v->arch.user_regs.ebp;
> -        c.cmp->user_regs.eax               = v->arch.user_regs.eax;
>          c.cmp->user_regs.eip               = v->arch.user_regs.eip;
>          c.cmp->user_regs.cs                = v->arch.user_regs.cs;
>          c.cmp->user_regs.eflags            = v->arch.user_regs.eflags;

Reviewed-by: Frediano Ziglio <[email protected]>

Frediano