[RFC/RFT PATCH 0/3] arm64: KVM: work around incoherency with uncached guest mappings

[Ard Biesheuvel]

On 24 February 2015 at 14:55, Andrew Jones <drjones at redhat.com> wrote:
> On Fri, Feb 20, 2015 at 04:36:26PM +0100, Andrew Jones wrote:
>> On Fri, Feb 20, 2015 at 02:37:25PM +0000, Ard Biesheuvel wrote:
>> > On 20 February 2015 at 14:29, Andrew Jones <drjones at redhat.com> wrote:
>> > > So looks like the 3 orders of magnitude greater number of traps
>> > > (only to el2) don't impact kernel compiles.
>> > >
>> >
>> > OK, good! That was what I was hoping for, obviously.
>> >
>> > > Then I thought I'd be able to quick measure the number of cycles
>> > > a trap to el2 takes with this kvm-unit-tests test
>> > >
>> > > int main(void)
>> > > {
>> > >         unsigned long start, end;
>> > >         unsigned int sctlr;
>> > >
>> > >         asm volatile(
>> > >         "       mrs %0, sctlr_el1\n"
>> > >         "       msr pmcr_el0, %1\n"
>> > >         : "=&r" (sctlr) : "r" (5));
>> > >
>> > >         asm volatile(
>> > >         "       mrs %0, pmccntr_el0\n"
>> > >         "       msr sctlr_el1, %2\n"
>> > >         "       mrs %1, pmccntr_el0\n"
>> > >         : "=&r" (start), "=&r" (end) : "r" (sctlr));
>> > >
>> > >         printf("%llx\n", end - start);
>> > >         return 0;
>> > > }
>> > >
>> > > after applying this patch to kvm
>> > >
>> > > diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
>> > > index bb91b6fc63861..5de39d740aa58 100644
>> > > --- a/arch/arm64/kvm/hyp.S
>> > > +++ b/arch/arm64/kvm/hyp.S
>> > > @@ -770,7 +770,7 @@
>> > >
>> > >         mrs     x2, mdcr_el2
>> > >         and     x2, x2, #MDCR_EL2_HPMN_MASK
>> > > -       orr     x2, x2, #(MDCR_EL2_TPM | MDCR_EL2_TPMCR)
>> > > +//     orr     x2, x2, #(MDCR_EL2_TPM | MDCR_EL2_TPMCR)
>> > >         orr     x2, x2, #(MDCR_EL2_TDRA | MDCR_EL2_TDOSA)
>> > >
>> > >         // Check for KVM_ARM64_DEBUG_DIRTY, and set debug to trap
>> > >
>> > > But I get zero for the cycle count. Not sure what I'm missing.
>> > >
>> >
>> > No clue tbh. Does the counter work as expected in the host?
>> >
>>
>> Guess not. I dropped the test into a module_init and inserted
>> it on the host. Always get zero for pmccntr_el0 reads. Or, if
>> I set it to something non-zero with a write, then I always get
>> that back - no increments. pmcr_el0 looks OK... I had forgotten
>> to set bit 31 of pmcntenset_el0, but doing that still doesn't
>> help. Anyway, I assume the problem is me. I'll keep looking to
>> see what I'm missing.
>>
>
> I returned to this and see that the problem was indeed me. I needed yet
> another enable bit set (the filter register needed to be instructed to
> count cycles while in el2). I've attached the code for the curious.
> The numbers are mean=6999, std_dev=242. Run on the host, or in a guest
> running on a host without this patch series (after TVM traps have been
> disabled), I get a pretty consistent 40.
>
> I checked how many vm-sysreg traps we do during the kernel compile
> benchmark. It's 124924. So it's a bit strange that we don't see the
> benchmark taking 10 to 20 seconds longer on average. I should probably
> double check my runs. In any case, while I like the approach of this
> series, the overhead is looking non-negligible.
>

Thanks a lot for producing these numbers. 125k x 7k == <1 billion
cycles == <1 second on a >1 GHz machine, I think?
Or am I missing something? How long does the actual compile take?

-- 
Ard.