Re: [PATCH v3 41/42] target/arm: Implement FEAT_HAFDBS

[Richard Henderson]

On 10/7/22 09:50, Peter Maydell wrote:
> On Fri, 7 Oct 2022 at 17:45, Richard Henderson
> <richard.henderson@linaro.org> wrote:
> > On 10/7/22 06:47, Peter Maydell wrote:
> > > Are there definitely no code paths where we might try to do
> > > a page table walk with the iothread already locked ?
> >
> > I'll double-check, but another possibility is to simply perform the atomic 
> > operation on
> > the low 32-bits, where both AF and DB are located.  Another trick I learned 
> > from x86...
>
> Doesn't that cause a problem where we don't detect that some other
> CPU wrote to the high 32 bits of the descriptor ? We're supposed to
> be using those high 32 bits, not the ones we have in hand...

Hmm, yes.  Which now makes me wonder if the x86 case is in fact buggy...

> If we do need the iothread lock, we could do it the way that
> io_readx() does, I guess, where we track whether we needed to
> lock it or not.

yes.


r~