Hi Mario, 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 > > 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: > pop x4, lr > pop x2, x3 > pop x0, x1 > @@ -754,9 +761,7 @@ __kvm_vcpu_return: > add x2, x0, #VCPU_CONTEXT > > save_guest_regs > - skip_fpsimd_state x3, 1f > bl __save_fpsimd > -1: > bl __save_sysregs > > skip_debug_state x3, 1f > @@ -777,9 +782,7 @@ __kvm_vcpu_return: > kern_hyp_va x2 > > bl __restore_sysregs > - skip_fpsimd_state x3, 1f > bl __restore_fpsimd > -1: > /* Clear FPSIMD and Trace trapping */ > msr cptr_el2, xzr > > Marc and I have hunted down the issue at KVM Forum and we believe we've found the issue. Please have a look at the following follow-up patch to Marc's patch above: diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S index 8b2a73b4..842e727 100644 --- a/arch/arm64/kvm/hyp.S +++ b/arch/arm64/kvm/hyp.S @@ -769,11 +769,26 @@ .macro activate_traps ldr x2, [x0, #VCPU_HCR_EL2] + + /* + * We are about to set CPTR_EL2.TFP to trap all floating point + * register accesses to EL2, however, the ARM ARM clearly states that + * traps are only taken to EL2 if the operation would not otherwise + * trap to EL1. Therefore, always make sure that for 32-bit guests, + * we set FPEXC.EN to prevent traps to EL1, when setting the TFP bit. + */ + tbnz x2, #HCR_RW_SHIFT, 99f // open code skip_32bit_state + mov x3, #(1 << 30) + msr fpexc32_el2, x3 + isb +99: + msr hcr_el2, x2 mov x2, #CPTR_EL2_TTA orr x2, x2, #CPTR_EL2_TFP msr cptr_el2, x2 + mov x2, #(1 << 15) // Trap CP15 Cr=15 msr hstr_el2, x2 Thanks, -Christoffer -- 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
