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Re: [PATCH 1/2] x86/svm: Remove regs param from asm-called functions
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 12 Aug 2022 09:06:57 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Fri, 12 Aug 2022 07:07:20 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 11.08.2022 21:59, Andrew Cooper wrote:
> A optimisation is going to want to conditionally have extra data on the stack
> around VMExit.
>
> We could alternative between `mov %rsp, %rdi` and `lea 8(%rsp), %rdi`, but it
> is easier just to make the functions void and let the compiler do the (not
> very) hard work.
>
> Passing regs is a bit weird for HVM guests anyway, because the resulting
> pointer is invariant (this isn't native exception handling where the regs
> pointers *are* important), and all functions calculate `current` themselves
> which is another invariant.
>
> Finally, the compiler can merge the get_cpu_info() calculation which is common
> to both `current` and guest_cpu_user_regs(), meaning the delta in C really is
> just one `lea`, and not any more expensive than `mov`'s in ASM anyway.
>
> No functional change.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
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