From: Sean Christopherson
> Sent: 09 February 2024 21:47
>
> On Fri, Feb 09, 2024, Linus Torvalds wrote:
> > Sean? Does this work for the case you noticed?
>
> Yep. You can quite literally see the effect of the asm(""). A "good" sequence
> directly propagates the result from the VMREAD's destination register to its
> final destination
>
> <+1756>: mov $0x280e,%r13d
> <+1762>: vmread %r13,%r13
> <+1766>: jbe 0x209fa <sync_vmcs02_to_vmcs12+1834>
> <+1768>: mov %r13,0xe8(%rbx)
>
> whereas the "bad" sequence bounces through a different register.
>
> <+1780>: mov $0x2810,%eax
> <+1785>: vmread %rax,%rax
> <+1788>: jbe 0x209e4 <sync_vmcs02_to_vmcs12+1812>
> <+1790>: mov %rax,%r12
> <+1793>: mov %r12,0xf0(%rbx)
...
Annoying, but I doubt it is measurable in this case.
Firstly it could easily be a 'free' register rename.
Secondly isn't vmread horribly slow anyway, so an extra
clock or two won't matter?
The double register move that OPTIMER_HIDE_VAR() often
generates is another matter entirely :-)
In the old days the peephole optimiser would (should?)
have removed most of these.
David
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