Re: [PATCH v5 10/24] KVM: arm64: Set up FGT for Partitioned PMU

[Oliver Upton <oupton@kernel.org>]

On Fri, Dec 12, 2025 at 08:51:34PM +0000, Colton Lewis wrote:
> > The other reason for doing this is kvm_pmu_fgt_bits() assumes a
> > 'positive' trap polarity, even though there are several cases where FGTs
> > have a 'negative' priority (i.e. 0 => trap).
> 
> For the bits I was concerned with they all had positive polarity, except
> for the dedicated instruction counter. (Side note: Why would ARM do
> this?)

Old software on new hardware, you don't want the guest to magically get
access to things it shouldn't.

> IIRC the FGT setup I plugged into in previous versions of the patch had
> some icky macros that accounted for polarity. They were confusing and I
> didn't like the effort to understand them.

I'm guessing you're referring to the undef infrastructure (FGUs), which
is a meaningfully load-bearing part of KVM.

> Is there a good reason not to adopt a convetion that 1 => trap for
> kernel code? Reversing the negative polarities immediately before write
> could be easy: Have a bitmap of the negative polarity bits to xor with
> the traps we actually want.

This *significantly* muddies the water around FGTs. I quite like that the
current representation matches the architecture. NV forces KVM to deal
with the native representations anyway.

Thanks,
Oliver