Re: [PATCH 1/9] linux-user: Diagnose incorrect -R size

[Peter Maydell]

On Mon, 6 Mar 2023 at 21:24, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> On 3/6/23 04:56, Peter Maydell wrote:
> > On Mon, 6 Mar 2023 at 02:14, Richard Henderson
> > <richard.henderson@linaro.org> wrote:
> >>
> >> Zero is the value for 'off', and should not be used with -R.
> >> We have been enforcing host page alignment for the non-R
> >> fallback of MAX_RESERVED_VA, but failing to enforce for -R.
> >
> > I'm pretty sure we have users who specifically use "-R 0" to
> > ask for "definitely turn off any reserved VA".
> > Here's a random example from an old gcc bug report:
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60681
> > and somebody using it via the environment variable:
> > https://www.openembedded.org/pipermail/openembedded-core/2015-January/100572.html
>
> Odd.
>
> Well, it won't actually have the effect of "definitely turn off", it will 
> merely leave
> things as per the default, which *will* enable reserved va for 32-bit guests 
> on 64-bit hosts.
>
> The only remaining question is whether we diagnose this oddness or silently 
> accept it.  It
> feels like someone playing with options they don't actually understand and an 
> error is
> warranted.

I'm pretty sure I've issued the advice "turn off the reserved
area stuff with -R 0" in the past, for working around various
QEMU bugs where it wasn't able to allocate the whole reserved
area it wanted to but the guest program didn't actually care.

thanks
-- PMM