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

[Ard Biesheuvel]

On 20 February 2015 at 14:29, Andrew Jones <drjones at redhat.com> wrote:
> On Thu, Feb 19, 2015 at 06:57:24PM +0100, Paolo Bonzini wrote:
>>
>>
>> On 19/02/2015 18:55, Andrew Jones wrote:
>> >> > > (I don't have an exact number for how many times it went to EL1 because
>> >> > >  access_mair() doesn't have a trace point.)
>> >> > > (I got the 62873 number by testing a 3rd kernel build that only had patch
>> >> > >  3/3 applied to the base, and counting kvm_toggle_cache events.)
>> >> > > (The number 50 is the number of kvm_toggle_cache events *without* 3/3
>> >> > >  applied.)
>> >> > >
>> >> > > I consider this bad news because, even considering it only goes to EL2,
>> >> > > it goes a ton more than it used to. I realize patch 3/3 isn't the final
>> >> > > plan for enabling traps though.
>>
>> If a full guest boots, can you try timing a kernel compile?
>>
>
> Guests boot. I used an 8 vcpu, 14G memory guest; compiled the kernel 4
> times inside the guest for each host kernel; base and mair. I dropped
> the time from the first run of each set, and captured the other 3.
> Command line used below. Time is from the
>   Elapsed (wall clock) time (h:mm:ss or m:ss):
> output of /usr/bin/time - the host's wall clock.
>
>   /usr/bin/time --verbose ssh $VM 'cd kernel && make -s clean && make -s -j8'
>
> Results:
> base: 3:06.11 3:07.00 3:10.93
> mair: 3:08.47 3:06.75 3:04.76
>
> 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?

-- 
Ard.