Hi Christoffer, On 06/08/15 12:54, Christoffer Dall wrote: > On Wed, Aug 05, 2015 at 05:11:37PM +0100, Marc Zyngier wrote: >> On 16/07/15 22:29, Mario Smarduch wrote: >>> This patch only saves and restores FP/SIMD registers on Guest access. To do >>> this cptr_el2 FP/SIMD trap is set on Guest entry and later checked on exit. >>> lmbench, hackbench show significant improvements, for 30-50% exits FP/SIMD >>> context is not saved/restored >>> >>> Signed-off-by: Mario Smarduch <m.smarduch@xxxxxxxxxxx> >> >> So this patch seems to break 32bit guests on arm64. I've had a look, >> squashed a few bugs that I dangerously overlooked during the review, but >> it still doesn't work (it doesn't crash anymore, but I get random >> illegal VFP instructions in 32bit guests). >> >> I'd be glad if someone could eyeball the following patch and tell me >> what's going wrong. If we don't find the root cause quickly enough, I'll >> have to drop the series from -next, and that'd be a real shame. >> >> Thanks, >> >> M. >> >> commit 5777dc55fbc170426a85e00c26002dd5a795cfa5 >> Author: Marc Zyngier <marc.zyngier@xxxxxxx> >> Date: Wed Aug 5 16:53:01 2015 +0100 >> >> KVM: arm64: NOTAFIX: Prevent crash when 32bit guest uses VFP >> >> Since we switch FPSIMD in a lazy way, access to FPEXC32_EL2 >> must be guarded by skip_fpsimd_state. Otherwise, all hell >> break loose. >> >> Also, FPEXC32_EL2 must be restored when we trap to EL2 to >> enable floating point. >> >> Note that while it prevents the host from catching fire, the >> guest still doesn't work properly, and I don't understand why just >> yet. >> >> Not-really-signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx> >> >> diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S >> index c8e0c70..b53ec5d 100644 >> --- a/arch/arm64/kvm/hyp.S >> +++ b/arch/arm64/kvm/hyp.S >> @@ -431,10 +431,12 @@ >> add x3, x2, #CPU_SYSREG_OFFSET(DACR32_EL2) >> mrs x4, dacr32_el2 >> mrs x5, ifsr32_el2 >> - mrs x6, fpexc32_el2 >> stp x4, x5, [x3] >> - str x6, [x3, #16] >> >> + skip_fpsimd_state x8, 3f >> + mrs x6, fpexc32_el2 >> + str x6, [x3, #16] >> +3: >> skip_debug_state x8, 2f >> mrs x7, dbgvcr32_el2 >> str x7, [x3, #24] >> @@ -461,10 +463,8 @@ >> >> add x3, x2, #CPU_SYSREG_OFFSET(DACR32_EL2) >> ldp x4, x5, [x3] >> - ldr x6, [x3, #16] >> msr dacr32_el2, x4 >> msr ifsr32_el2, x5 >> - msr fpexc32_el2, x6 >> >> skip_debug_state x8, 2f >> ldr x7, [x3, #24] >> @@ -669,12 +669,14 @@ __restore_debug: >> ret >> >> __save_fpsimd: >> + skip_fpsimd_state x3, 1f >> save_fpsimd >> - ret >> +1: ret >> >> __restore_fpsimd: >> + skip_fpsimd_state x3, 1f >> restore_fpsimd >> - ret >> +1: ret >> >> switch_to_guest_fpsimd: >> push x4, lr >> @@ -682,6 +684,7 @@ switch_to_guest_fpsimd: >> mrs x2, cptr_el2 >> bic x2, x2, #CPTR_EL2_TFP >> msr cptr_el2, x2 >> + isb > > ah, EL2 accesses themselves trap too, ouch. Yeah, the FP architecture has all kind of nasty tricks like this. >> >> mrs x0, tpidr_el2 >> >> @@ -692,6 +695,10 @@ switch_to_guest_fpsimd: >> add x2, x0, #VCPU_CONTEXT >> bl __restore_fpsimd >> >> + skip_32bit_state x3, 1f >> + ldr x4, [x2, #CPU_SYSREG_OFFSET(FPEXC32_EL2)] >> + msr fpexc32_el2, x4 >> +1: > > wait, it looks like you're missing a store of the host fpsimd state in > the switch_to_guest_fpsimd situation. A store of FPEXC32_EL2 for the host? No, this register has no significance whatsoever for a 64bit host. Its sole purpose is to accommodate a 32bit guest (see D7.2.31). Or are you thinking of something else? > It think this would be easier to follow if the aarch32 FP registers were > handled as part of the __save_fpsimd and __restore_fpsimd routines > instead. Maybe. This code needs some rework... > Does this help anything? Not really, sorry... :-( > Otherwise, I assume you've tested thoroughly between something like > v4.2-rc2 and this patch, so that you're sure we didn't have the 32-bit > processed crash before? Without the lazy switching, we seem to be rock solid. With lazy switching, I get these illegal instructions, always on VFP ops. My current thinking is that it has something to do with CPACR_EL1, and the fact that the 32bit kernel turns it VFP on/off all the time. /me puzzled... 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
