Re: [PATCH][UPDATE] shortcut for lightweight VM Exit

Avi Kivity
2007-04-30T13:10:47Z

Dong, Eddie wrote:
> Avi Kivity wrote:
>
>> Dong, Eddie wrote:
>>
>>> This patch provides short cut handling for light weight VM Exit,
>>> which can boost KB performance 11% under FC5 guest.
>>> Any comments?
>>>
>> While the patch looks good, it kills FC6 x86_64 boot with a double
>> fault. Guest kernel is 2.6.18-1.2798. dmesg says
>>
>> inject_page_fault: double fault 0x7fff79b58fd8 @ 0xffffffff8025cca1
>>
>>
> I think I know the reason, the SYSCALL_MASK & LSTAR update in guest
> needs to
> update the physical side MSR after this patch. With this update in
> vmx_set_msr,
> I get it up on a RHEL5U 64 bits guest but no time to check the detail
> performance
> gain yet. (will report after May day national holiday 5/1--5/8 in PRC)
>
> BTW, I have another patch in hand to further reduce MSR save/restore and
> thus
> improve performance more for lightweight VM Exit. Base on my observation
> for FC5 32 bits
> guest, 93% VM Exit will fall into the lightweight path.
>
>

I measured 650 cycles saved from a total of 5300 before the patch.

The patch had a bug where it would exit to userspace if need_resched()
was true. This can reduce performance on a high context switch scenario
(many virtual machines). I fixed that, and also a minor coding style
issue, and applied.

Enjoy your holiday :)

--
error compiling committee.c: too many arguments to function