Re: [RFC PATCH v2] arm/ptw: Handle atomic updates of page tables entries in MMIO during PTW.

[Peter Maydell]

On Fri, 23 Feb 2024 at 10:02, Peter Maydell <peter.maydell@linaro.org> wrote:
>
> On Thu, 22 Feb 2024 at 21:21, Richard Henderson
> <richard.henderson@linaro.org> wrote:
> >
> > On 2/19/24 06:12, Jonathan Cameron wrote:
> > > I'm far from confident this handling here is correct. Hence
> > > RFC.  In particular not sure on what locks I should hold for this
> > > to be even moderately safe.
> > >
> > > The function already appears to be inconsistent in what it returns
> > > as the CONFIG_ATOMIC64 block returns the endian converted 'eventual'
> > > value of the cmpxchg whereas the TCG_OVERSIZED_GUEST case returns
> > > the previous value.
>
> I think this is a bug in the TCG_OVERSIZED_GUEST handling,
> which we've never noticed because you only see that case
> on a 32-bit host.

Having looked through the code and talked to Richard on IRC,
I think that the TCG_OVERSIZED_GUEST handling is correct.
The return value of qatomic_cmpxchg__nocheck() is the value
that was in memory before the cmpxchg. So the TCG_OVERSIZED_GUEST
code's semantics are the same as the normal path. (The comment on
qatomic_cmpxchg__nocheck() that describes this as the
"eventual value" is rather confusing so we should improve that.)

thanks
-- PMM