Re: [Xen-devel] [PATCH v7 15/17] vmx: VT-d posted-interrupt core logic handling

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.09.15 at 03:50, <feng.wu@xxxxxxxxx> wrote:
> One issue is the number of vmexits is far far bigger than the number of
> context switch. I test it for a quite short time and it shows there are
> 2910043 vmexits and 71733 context switch (only count the number in
> __context_switch() since we only change the PI state in this function).
> If we change the PI state in each vmexit/vmentry, I am afraid this will
> hurt the performance.

Note that George has already asked whether the updating of the
PI descriptor is expensive, without you answering. If this is basically
just a memory or VMCS field write, I don't think it really matters in
which code path it sits, regardless of the frequency of either path
being used. Also note that whatever measuring you do in an area
like this, it'll only be an example, unlikely to be representative of
anything. Plus with the tendency to eliminate VMEXITs with newer
hardware, the penalty of this sitting in the VMEXIT path ought to go
down.

Jan



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