Re: [RFC/HACK] x86: Fast return to kernel

[H. Peter Anvin]

On 05/04/2014 04:46 PM, Paolo Bonzini wrote:
> 
> Your suggested trick of splitting the return paths for IF=0/IF=1 can be
> also done like this:
> 
>     movq EFLAGS-ARGOFFSET(%rsp), %rdi
>     btrq $9, %rdi        # Clear IF, save old value in CF
>     movq %rdi, (%rsi)
>     ...
>     popfq
>     jnc    1f        # If IF was 0, just return
>     sti            # Using STI gets us an interrupt shadow
> 1f:
>     retq
> 

That doesn't work, because CF gets restored by the popfq as well.
Unfortunately.

	-hpa


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