Re: [PATCH 00/11] KVM: arm64: Add NV timer support

Chase Conklin
2024-12-09T14:24:32Z

Hi Marc,

On Mon, 2 Dec 2024 17:21:23 +0000, Marc Zyngier <maz@kernel.org> wrote:
> Here's another batch of NV-related patches, this time bringing in most
> of the timer support for EL2 as well as nested guests.
>
> The code is pretty convoluted for a bunch of reasons:
>
> - FEAT_NV2 breaks the timer semantics by redirecting HW controls to
> memory, meaning that a guest could setup a timer and never see it
> firing until the next exit
>
> - We go try hard to reflect the timer state in memory, but that's not
> great.
>
> - With FEAT_ECV, we can finally correctly emulate the virtual timer,
> but this emulation is pretty costly
>
> - As a way to make things suck less, we handle timer reads as early as
> possible, and only defer writes to the normal trap handling
>
> - Finally, some implementations are badly broken, and require some
> hand-holding, irrespective of NV support. So we try and reuse the NV
> infrastructure to make them usable. This could be further optimised,
> but I'm running out of patience for this sort of HW.
>
> What this is not implementing is support for CNTPOFF_EL2. It appears
> that the architecture doesn't let you correctly emulate it, so I guess
> this will be trap/emulate for the foreseeable future.
>
> This series is on top of v6.13-rc1, and has been tested on my usual M2
> setup, but also on a Snapdragon X1 Elite devkit. I would like to thank
> Qualcomm for the free hardware with no strings (nor support) attached!
>
> If you are feeling brave, you can run the whole thing from [1].
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=kvm-arm64/nv-next
>

I was feeling brave, and I think I see an issue in the cpufeature change
in the kvm-arm64/nv-e2h-select branch that's a part of
kvm-arm64/nv-next. In d75a4820a897 ("arm64: cpufeature: Handle NV_frac as a synonym of NV2"),
I don't see NV_frac being added to the FTR bits. I believe that means it
will get sanitized out and consequently not seen by the NV feature
detection code. Does that commit also need:

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 9fa8bd77ae0..f97459e160b 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -480,6 +480,7 @@ static const struct arm64_ftr_bits ftr_id_aa64mmfr3[] = {

static const struct arm64_ftr_bits ftr_id_aa64mmfr4[] = {
S_ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64MMFR4_EL1_E2H0_SHIFT, 4, 0),
+ S_ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64MMFR4_EL1_NV_frac_SHIFT, 4, 0),
ARM64_FTR_END,
};

Thanks,
Chase