[RFC/RFT PATCH 0/3] arm64: KVM: work around incoherency with uncached guest mappings
From: Ard Biesheuvel <hidden>
Date: 2015-02-20 14:37:25
Also in:
kvm, kvmarm
On 20 February 2015 at 14:29, Andrew Jones [off-list ref] wrote:
On Thu, Feb 19, 2015 at 06:57:24PM +0100, Paolo Bonzini wrote:quoted
On 19/02/2015 18:55, Andrew Jones wrote:quoted
quoted
quoted
quoted
(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.
quoted hunk ↗ jump to hunk
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 kvmdiff --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 trapBut 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.