Re: [PATCH 24/29] arm64: KVM: 32bit conditional execution emulation

Christopher Covington
2013-03-18T17:04:02Z

Hi Marc,

On 03/04/2013 10:47 PM, Marc Zyngier wrote:
> As conditionnal instructions can trap on AArch32, add the thinest

conditional

> possible emulation layer to keep 32bit guests happy.

[...]

> diff --git a/arch/arm64/kvm/emulate.c b/arch/arm64/kvm/emulate.c
> new file mode 100644
> index 0000000..6b3dbc3
> --- /dev/null
> +++ b/arch/arm64/kvm/emulate.c
> @@ -0,0 +1,154 @@
> +/*
> + * (not much of an) Emulation layer for 32bit guests.
> + *
> + * Copyright (C) 2012 - Virtual Open Systems and Columbia University
> + * Author: Christoffer Dall <c.dall@virtualopensystems.com>
> + *
> + * This program is free software: you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 as
> + * published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program. If not, see <http://www.gnu.org/licenses/>.
> + */
> +
> +#include <linux/kvm_host.h>
> +#include <asm/kvm_emulate.h>
> +
> +/*
> + * stolen from arch/arm/kernel/opcodes.c
> + *
> + * condition code lookup table
> + * index into the table is test code: EQ, NE, ... LT, GT, AL, NV
> + *
> + * bit position in short is condition code: NZCV
> + */
> +static const unsigned short cc_map[16] = {
> + 0xF0F0, /* EQ == Z set */
> + 0x0F0F, /* NE */
> + 0xCCCC, /* CS == C set */
> + 0x3333, /* CC */
> + 0xFF00, /* MI == N set */
> + 0x00FF, /* PL */
> + 0xAAAA, /* VS == V set */
> + 0x5555, /* VC */
> + 0x0C0C, /* HI == C set && Z clear */
> + 0xF3F3, /* LS == C clear || Z set */
> + 0xAA55, /* GE == (N==V) */
> + 0x55AA, /* LT == (N!=V) */
> + 0x0A05, /* GT == (!Z && (N==V)) */
> + 0xF5FA, /* LE == (Z || (N!=V)) */
> + 0xFFFF, /* AL always */
> + 0 /* NV */
> +};
> +
> +static int kvm_vcpu_get_condition(struct kvm_vcpu *vcpu)
> +{
> + u32 esr = kvm_vcpu_get_hsr(vcpu);
> +
> + if (esr & ESR_EL2_CV)
> + return (esr & ESR_EL2_COND) >> ESR_EL2_COND_SHIFT;
> +
> + return -1;
> +}
> +
> +/*
> + * Check if a trapped instruction should have been executed or not.
> + */
> +bool kvm_condition_valid32(struct kvm_vcpu *vcpu)
> +{
> + unsigned long cpsr;
> + u32 cpsr_cond;
> + int cond;
> +
> + /* Top two bits non-zero? Unconditional. */
> + if (kvm_vcpu_get_hsr(vcpu) >> 30)
> + return true;
> +
> + /* Is condition field valid? */
> + cond = kvm_vcpu_get_condition(vcpu);
> + if (cond == 0xE)
> + return true;
> +
> + cpsr = *vcpu_cpsr(vcpu);
> +
> + if (cond < 0) {
> + /* This can happen in Thumb mode: examine IT state. */
> + unsigned long it;
> +
> + it = ((cpsr >> 8) & 0xFC) | ((cpsr >> 25) & 0x3);
> +
> + /* it == 0 => unconditional. */
> + if (it == 0)
> + return true;
> +
> + /* The cond for this insn works out as the top 4 bits. */
> + cond = (it >> 4);
> + }
> +
> + cpsr_cond = cpsr >> 28;
> +
> + if (!((cc_map[cond] >> cpsr_cond) & 1))
> + return false;
> +
> + return true;
> +}
> +
> +/**
> + * adjust_itstate - adjust ITSTATE when emulating instructions in IT-block
> + * @vcpu: The VCPU pointer
> + *
> + * When exceptions occur while instructions are executed in Thumb IF-THEN
> + * blocks, the ITSTATE field of the CPSR is not advanved (updated), so we have
> + * to do this little bit of work manually. The fields map like this:
> + *
> + * IT[7:0] -> CPSR[26:25],CPSR[15:10]
> + */
> +static void kvm_adjust_itstate(struct kvm_vcpu *vcpu)
> +{
> + unsigned long itbits, cond;
> + unsigned long cpsr = *vcpu_cpsr(vcpu);
> + bool is_arm = !(cpsr & COMPAT_PSR_T_BIT);
> +
> + BUG_ON(is_arm && (cpsr & COMPAT_PSR_IT_MASK));
> +
> + if (!(cpsr & COMPAT_PSR_IT_MASK))
> + return;
> +
> + cond = (cpsr & 0xe000) >> 13;
> + itbits = (cpsr & 0x1c00) >> (10 - 2);
> + itbits |= (cpsr & (0x3 << 25)) >> 25;
> +
> + /* Perform ITAdvance (see page A-52 in ARM DDI 0406C) */
> + if ((itbits & 0x7) == 0)
> + itbits = cond = 0;
> + else
> + itbits = (itbits << 1) & 0x1f;
> +
> + cpsr &= ~COMPAT_PSR_IT_MASK;
> + cpsr |= cond << 13;
> + cpsr |= (itbits & 0x1c) << (10 - 2);
> + cpsr |= (itbits & 0x3) << 25;
> + *vcpu_cpsr(vcpu) = cpsr;
> +}

Maybe I'm spoiled by the breadth of the 64-bit definitions, but I wonder if
fewer magic numbers in the 32-bit emulation might make it an easier read.

Christopher

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