Re: [PATCH v2 0/4] KVM: arm64: vcpu sysreg accessor rework

[Marc Zyngier <maz@kernel.org>]

On Wed, 04 Jun 2025 16:53:01 +0100,
Miguel Luis <miguel.luis@oracle.com> wrote:
> 
> 
> 
> > On 4 Jun 2025, at 15:17, Marc Zyngier <maz@kernel.org> wrote:
> > 
> > On Wed, 04 Jun 2025 11:47:57 +0100,
> > Miguel Luis <miguel.luis@oracle.com> wrote:
> >> 
> >> Hi Marc,
> >> 
> >>> On 3 Jun 2025, at 07:08, Marc Zyngier <maz@kernel.org> wrote:
> >>> 
> >>> This series tries to bring some sanity to the way the RESx masks
> >>> are applied when accessing the in-memory view of the guest's
> >>> system registers.
> >>> 
> >>> Currently, we have *one* accessor (__vcpu_sys_reg()) that can either
> >>> be used as a rvalue or lvalue while that applies the RESx masks behind
> >>> the scenes. This works fine when used as a rvalue.
> >>> 
> >>> However, when used as a lvalue, it does the wrong thing, as it only
> >>> sanitises the value we're about to overwrite. This is pointless work
> >>> and potentially hides bugs.
> >>> 
> >>> I propose that we move to a set of store-specific accessors (for
> >>> assignments and RMW) instead of the lvalue hack, ensuring that the
> >>> assigned value is the one that gets sanitised. This then allows the 
> >>> legacy accessor to be converted to rvalue-only.
> >>> 
> >>> Given the level of churn this introduces, I'd like this to land very
> >>> early in the cycle. Either before 6.16-rc2, or early in 6.17.
> >>> 
> >> 
> >> For the series:
> >> Reviewed-by: Miguel Luis <miguel.luis@oracle.com>
> > 
> > Thanks.
> > 
> >> 
> >> nit: the rmw accessor implies an implicit assignment which could be specified
> >> within its macro instead but it's fine by me.
> > 
> > I'm not sure what you mean. Looking at this macro again, the early
> > writeback to the sysreg array is pretty unfortunate, and could be
> > avoided.
> > 
> > Does the following change address your concern? If not, please clearly
> > point out what you're after.
> 
> I believe this should give a clearer idea while avoids repeating the assignment everywhere
> the accessor is used. Just replace ‘&=‘ and ‘|=‘ by ‘&’ and ‘|’.

I have the opposite view. I think the '=' sign conveys an important
meaning, and hiding it makes things less obvious. If I had lost common
sense and was writing C++ , I would have simply overloaded the various
operators, retaining the existing syntax. This macro does the hard job
of applying the RESx masks, but the operator is what is semantically
significant.

Thanks,

	M.

-- 
Jazz isn't dead. It just smells funny.