[PATCH] arm64: KVM: Optimize arm64 guest exit VFP/SIMD register save/restore

Marc Zyngier marc.zyngier at arm.com
Mon Jun 15 11:20:21 PDT 2015

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).


>>>  	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.


Jazz is not dead. It just smells funny...

More information about the linux-arm-kernel mailing list