On 15/06/15 19:44, Mario Smarduch wrote: > On 06/15/2015 11:20 AM, Marc Zyngier wrote: >> On 15/06/15 19:04, Mario Smarduch wrote: >>> On 06/15/2015 03:00 AM, Marc Zyngier wrote: >>>> Hi Mario, >>>> >>>> I was working on a more ambitious patch series, >>>> but we probably ought to >>>> start small, and this looks fairly sensible to me. >>> >>> Hi Marc, >>> thanks for reviewing, I was thinking to post this >>> first and next iteration on guest access switch >>> back to host registers only upon return to user space or >>> vCPU context switch. This should save more cycles for >>> various exits. >>> >>> Were you thinking along the same lines or something >>> altogether different? >> >> That's mostly what I had in mind. Basically staying away from touching >> the FP registers until vcpu_put(). I had it mostly working, but >> experienced some interesting corruption cases, specially when using >> 32bit guests. >> >>> >>>> >>>> A few minor comments below. >>>> >>>> On 13/06/15 23:20, Mario Smarduch wrote: >>>>> Currently VFP/SIMD registers are always saved and restored >>>>> on Guest entry and exit. >>>>> >>>>> This patch only saves and restores VFP/SIMD registers on >>>>> Guest access. To do this cptr_el2 VFP/SIMD trap is set >>>>> on Guest entry and later checked on exit. This follows >>>>> the ARMv7 VFPv3 implementation. Running an informal test >>>>> there are high number of exits that don't access VFP/SIMD >>>>> registers. >>>> >>>> It would be good to add some numbers here. How often do we exit without >>>> having touched the FPSIMD regs? For which workload? >>> >>> Lmbench is what I typically use, with ssh server, i.e., cause page >>> faults and interrupts - usually registers are not touched. >>> I'll run the tests again and define usually. >>> >>> Any other loads you had in mind? >> >> Not really (apart from running hackbench, of course...;-). I'd just like >> to see the numbers in the commit message, so that we can document the >> improvement (and maybe track regressions). > > Ok I understand. > >> >> [...] >> >>>> >>>>> skip_debug_state x3, 1f >>>>> // Clear the dirty flag for the next run, as all the state has >>>>> // already been saved. Note that we nuke the whole 64bit word. >>>>> @@ -1166,6 +1211,10 @@ el1_sync: // Guest trapped into EL2 >>>>> mrs x1, esr_el2 >>>>> lsr x2, x1, #ESR_ELx_EC_SHIFT >>>>> >>>>> + /* Guest accessed VFP/SIMD registers, save host, restore Guest */ >>>>> + cmp x2, #ESR_ELx_EC_FP_ASIMD >>>>> + b.eq switch_to_guest_vfp >>>>> + >>>> >>>> I'd prefer you moved that hunk to el1_trap, where we handle all the >>>> traps coming from the guest. >>> >>> I'm thinking would it make sense to update the armv7 side as >>> well. When reading both exit handlers the flow mirrors >>> each other. >> >> The 32bit code is starting to show its age, and could probably do with a >> refactor. If you have some cycles to spare, that'd be quite interesting. > > Yep, will do, ARMv7 is still very relevant. You bet it is. My home router is a v7 VM... M. -- Jazz is not dead. It just smells funny... -- 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
