Re: [Xen-devel] [PATCH] x86/xpti: avoid copying L4 page table contents when possible

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.02.18 at 10:03, <jgross@xxxxxxxx> wrote:
> On 20/02/18 09:56, Jan Beulich wrote:
>>>>> On 19.02.18 at 18:19, <jgross@xxxxxxxx> wrote:
>>> On 19/02/18 17:59, Jan Beulich wrote:
>>>>>>> On 15.02.18 at 13:52, <jgross@xxxxxxxx> wrote:
>>>>> --- a/xen/arch/x86/x86_64/entry.S
>>>>> +++ b/xen/arch/x86/x86_64/entry.S
>>>>> @@ -46,10 +46,13 @@ restore_all_guest:
>>>>>  .Lrag_cr3_start:
>>>>>          mov   VCPU_cr3(%rbx), %r9
>>>>>          GET_STACK_END(dx)
>>>>> -        mov   STACK_CPUINFO_FIELD(pv_cr3)(%rdx), %rdi
>>>>> +        mov   STACK_CPUINFO_FIELD(pv_cr3)(%rdx), %rax
>>>>> +        cmpb  $0, STACK_CPUINFO_FIELD(copy_l4)(%rdx)
>>>>> +        je    .Lrag_copyend
>>>>> +        movb  $0, STACK_CPUINFO_FIELD(copy_l4)(%rdx)
>>>>
>>>> Use %bl instead of $0 in both cases (with a comment)?
>>>
>>> Do you really think we should add such a micro optimization relying
>>> on the vcpu pointer being aligned to page boundary? And are you sure
>>> adding the register dependency isn't hurting more than we will gain
>>> from saving an instruction byte?
>> 
>> Well, both of what you say are the precise reasons why I've
>> asked a question rather than asking for the change to be done
>> unconditionally. I'm not really worried about the register
>> dependency - %rbx is loaded sufficiently much earlier. And
>> the struct-vcpu-is-aligned aspect is why I did suggest adding
>> a comment; I assume you realize we can't easily break that
>> alignment, hence I see no significant risk here. But I'm not
>> going to insist, and I'm open for further counter arguments.
> 
> What about the following idea: instead of special casing this single
> instance of replacing $0 with a register we could e.g. replace all
> %rbx uses by %r13 and set %rbx to 0 globally. This would enable us
> to replace _all_ $0 uses by %rbx/%ebx/%bx/%bl.

I dislike this, both because use of %r13 will require a REX prefix
even on insns that currently don't need one, and because I
dislike wasting a register just to hold zero. It's no like we have
that many callee-saved registers left, now that we had to start
using some of them beyond %rbx and %rbp.

Jan


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