Re: [PATCH v5 6/7] KVM: arm64: Mask FEAT_CCIDX

From: Akihiko Odaki
Date: Sat Jan 07 2023 - 04:54:57 EST


On 2023/01/06 7:22, Oliver Upton wrote:
On Fri, Dec 30, 2022 at 06:54:51PM +0900, Akihiko Odaki wrote:
The CCSIDR access handler masks the associativity bits according to the
bit layout for processors without FEAT_CCIDX. KVM also assumes CCSIDR is
32-bit where it will be 64-bit if FEAT_CCIDX is enabled. Mask FEAT_CCIDX
so that these assumptions hold.

Suggested-by: Marc Zyngier <maz@xxxxxxxxxx>
Signed-off-by: Akihiko Odaki <akihiko.odaki@xxxxxxxxxx>

FYI, I'm an idiot and replied to v4 of this patch... Forwarding comments
below:

---
arch/arm64/kvm/sys_regs.c | 11 +++++++++++
1 file changed, 11 insertions(+)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index f4a7c5abcbca..aeabf1f3370b 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1124,6 +1124,12 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu, struct sys_reg_desc const *r
ID_DFR0_PERFMON_SHIFT,
kvm_vcpu_has_pmu(vcpu) ? ID_DFR0_PERFMON_8_4 : 0);
break;
+ case SYS_ID_AA64MMFR2_EL1:
+ val &= ~ID_AA64MMFR2_EL1_CCIDX_MASK;
+ break;
+ case SYS_ID_MMFR4_EL1:
+ val &= ~ARM64_FEATURE_MASK(ID_MMFR4_CCIDX);
+ break;

Not that it is necessarily worth addressing, but I wanted to point
something out.

This change breaks migration from older kernels on implementations w/
FEAT_CCIDX. There is most likely exactly 0 of those in the wild, but
we need to be careful changing user-visible stuff like this.

--
Thanks,
Oliver

I also don't think whether FEAT_CCIDX is visible matters for any guest because the line size a guest would care is held in the same bits whether FEAT_CCIDX is implemented. But if it concerns you I can write a bit more code so that it won't mask CCIDX bit if it's set from the userspace.

Regards,
Akihiko Odaki