[RFC PATCH 2/2] arm64: kvm: describe data or unified caches as having 1 set and 1 way

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

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.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 arch/arm64/kvm/sys_regs.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 464e794b5bc5..eb244ff98dca 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1180,6 +1180,21 @@ 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, 2);
 	return true;
 }
 
-- 
2.17.1


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