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

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>

2 files changed, 17 insertions, 1 deletions

diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
index 9acccb1e3746..d3842791e1c4 100644
--- a/arch/arm64/include/asm/kvm_emulate.h
+++ b/arch/arm64/include/asm/kvm_emulate.h
@@ -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;
 }
 
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 45a07cfc57ed..81a342679e60 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -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;
 }