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arm64: KVM: Describe data or unified caches as having 1 set and 1 way

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On SMP ARM systems, cache maintenance by set/way should only ever be
done in the context of onlining or offlining CPUs, which is typically
done by bare metal firmware and never in a virtual machine. For this
reason, we trap set/way cache maintenance operations and replace them
with conditional flushing of the entire guest address space.

Due to this trapping, the set/way arguments passed into the set/way
ops are completely ignored, and thus irrelevant. This also means that
the set/way geometry is equally irrelevant, and we can simply report
it as 1 set and 1 way, so that legacy 32-bit ARM system software (i.e.,
the kind that only receives odd fixes) doesn't take a performance hit
due to the trapping when iterating over the cachelines.

Acked-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
This commit is contained in:
Ard Biesheuvel 2019-01-31 14:17:18 +01:00 committed by Marc Zyngier
parent f7f2b15c3d
commit 793acf870e
2 changed files with 17 additions and 1 deletions
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3
arch/arm64/include/asm/kvm_emulate.h
View File

@ -78,7 +78,8 @@ static inline void vcpu_reset_hcr(struct kvm_vcpu *vcpu)
if (!vcpu_el1_is_32bit(vcpu))
vcpu->arch.hcr_el2 |= HCR_TID3;
if (cpus_have_const_cap(ARM64_MISMATCHED_CACHE_TYPE))
if (cpus_have_const_cap(ARM64_MISMATCHED_CACHE_TYPE) ||
vcpu_el1_is_32bit(vcpu))
vcpu->arch.hcr_el2 |= HCR_TID2;
}

15
arch/arm64/kvm/sys_regs.c
View File

@ -1186,6 +1186,21 @@ static bool access_ccsidr(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
csselr = vcpu_read_sys_reg(vcpu, CSSELR_EL1);
p->regval = get_ccsidr(csselr);
/*
* Guests should not be doing cache operations by set/way at all, and
* for this reason, we trap them and attempt to infer the intent, so
* that we can flush the entire guest's address space at the appropriate
* time.
* To prevent this trapping from causing performance problems, let's
* expose the geometry of all data and unified caches (which are
* guaranteed to be PIPT and thus non-aliasing) as 1 set and 1 way.
* [If guests should attempt to infer aliasing properties from the
* geometry (which is not permitted by the architecture), they would
* only do so for virtually indexed caches.]
*/
if (!(csselr & 1)) // data or unified cache
p->regval &= ~GENMASK(27, 3);
return true;
}