On 05/04/2010 11:15 AM, Suresh Siddha wrote: > On Tue, 2010-05-04 at 00:41 -0700, Avi Kivity wrote: >> On 05/04/2010 12:45 AM, H. Peter Anvin wrote: >>> >>>> I was trying to avoid a performance regression relative to the current >>>> code, as it appears that some care was taken to avoid the memory reference. >>>> >>>> I agree that it's probably negligible compared to the save/restore >>>> code. If the x86 maintainers agree as well, I'll replace it with >>>> cpu_has_xsave. >>>> >>>> >>> I asked Suresh to comment on this, since he wrote the original code. He >>> did confirm that the intent was to avoid a global memory reference. >>> >>> >> >> Ok, so you're happy with the patch as is? > > As use_xsave() is in the hot context switch path, I would like to go > with Avi's proposal. > I would tend to agree. Saving a likely cache miss in the hot context switch path is worthwhile. I would like to request one change, however. I would like to see the alternatives code to be: movb $0,reg movb $1,reg ... instead of using xor (which has to be padded with NOPs, which is of course pointless since the slot is a fixed size.) I would suggest using a byte-sized variable instead of a dword-size variable to save a few bytes, too. Once the jump label framework is integrated and has matured, I think we should consider using it to save the mov/test/jump. -hpa -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
