On Mon, Nov 10, 2014 at 2:45 AM, Gleb Natapov <gleb@xxxxxxxxxx> wrote: > On Mon, Nov 10, 2014 at 11:03:35AM +0100, Paolo Bonzini wrote: >> >> >> On 09/11/2014 17:36, Andy Lutomirski wrote: >> >> The purpose of vmexit test is to show us various overheads, so why not >> >> measure EFER switch overhead by having two tests one with equal EFER >> >> another with different EFER, instead of hiding it. >> > >> > I'll try this. We might need three tests, though: NX different, NX >> > same but SCE different, and all flags the same. >> >> The test actually explicitly enables NX in order to put itself in the >> "common case": >> >> commit 82d4ccb9daf67885a0316b1d763ce5ace57cff36 >> Author: Marcelo Tosatti <mtosatti@xxxxxxxxxx> >> Date: Tue Jun 8 15:33:29 2010 -0300 >> >> test: vmexit: enable NX >> >> Enable NX to disable MSR autoload/save. This is the common case anyway. >> >> Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx> >> Signed-off-by: Avi Kivity <avi@xxxxxxxxxx> >> >> (this commit is in qemu-kvm.git), so I guess forgetting to set SCE is >> just a bug. The results on my Xeon Sandy Bridge are very interesting: >> >> NX different ~11.5k (load/save EFER path) >> NX same, SCE different ~19.5k (urn path) >> all flags the same ~10.2k >> >> The inl_from_kernel results have absolutely no change, usually at most 5 >> cycles difference. This could be because I've added the SCE=1 variant >> directly to vmexit.c, so I'm running the tests one next to the other. >> >> I tried making also the other shared MSRs the same between guest and >> host (STAR, LSTAR, CSTAR, SYSCALL_MASK), so that the user return notifier >> has nothing to do. That saves about 4-500 cycles on inl_from_qemu. I >> do want to dig out my old Core 2 and see how the new test fares, but it >> really looks like your patch will be in 3.19. >> > Please test on wide variety of HW before final decision. Also it would > be nice to ask Intel what is expected overhead. It is awesome if they > mange to add EFER switching with non measurable overhead, but also hard > to believe :) Also Andy had an idea do disable switching in case host > and guest EFERs are the same but IIRC his patch does not include it yet. I'll send that patch as a followup in a sec. It doesn't seem to make a difference, which reinforces my hypothesis that microcode is fiddling with EFER on entry and exit anyway to handle LME and LMA anyway, so adjusting the other bits doesn't effect performance. --Andy > > -- > Gleb. -- Andy Lutomirski AMA Capital Management, LLC -- 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
