Re: [PATCH] tcg/arm: Increase stack alignment for function generation

[Richard W.M. Jones]

On Thu, Sep 02, 2021 at 08:36:56AM +0100, Peter Maydell wrote:
> On Wed, 1 Sept 2021 at 21:24, Richard W.M. Jones <rjones@redhat.com> wrote:
> >
> > On Wed, Sep 01, 2021 at 09:17:07PM +0100, Peter Maydell wrote:
> > > On Wed, 1 Sept 2021 at 19:51, Richard W.M. Jones <rjones@redhat.com> 
> > > wrote:
> > > >
> > > > On Wed, Sep 01, 2021 at 07:41:21PM +0100, Peter Maydell wrote:
> > > > > Is the failure case short enough to allow -d ... logging to
> > > > > be taken? That's usually the most useful info, but it's so huge
> > > > > it's often not feasible.
> > > >
> > > > I can try -- what exact -d option would be useful?
> > >
> > > Depends what you're after. Personally I'm fairly sure I know
> > > what's going on, I'm just not sure what the right fix is.
> >
> > Another question: We couldn't reproduce this even with the identical
> > ARM guest kernel + initrd + command line using qemu-system-arm
> > compiled for x86-64 host.  This was a bit surprising!  Was that bad
> > luck or is there some reason why this bug might not be reproducible
> > except on armv7 host?  (Both cases use -machine accel=tcg).
> 
> That's expected -- this is a bug in the codegen for arm hosts
> (specifically 32-bit arm where Neon is available). tcg/i386/
> sets TCG_TARGET_STACK_ALIGN to 16, so it won't hit the assert.
> 
> Yesterday I wrote:
> > The prologue does seem to actively align to the
> > specified value, not merely assume-and-preserve that alignment.
> 
> but I was misreading the code -- it does just assume-and-preserve.
> 
> Do you need an urgent fix/workaround for this? The simplest thing
> is to wait for RTH to look at this, which is not likely to be before
> the 13th.

We can wait for Richard to take a look.

Thanks,

Rich.

> Otherwise I think you can work around it with:
> 
> --- a/tcg/arm/tcg-target.h
> +++ b/tcg/arm/tcg-target.h
> @@ -152,7 +152,7 @@ extern bool use_neon_instructions;
>  #define TCG_TARGET_HAS_qemu_st8_i32     0
> 
>  #define TCG_TARGET_HAS_v64              use_neon_instructions
> -#define TCG_TARGET_HAS_v128             use_neon_instructions
> +#define TCG_TARGET_HAS_v128             0
>  #define TCG_TARGET_HAS_v256             0
> 
>  #define TCG_TARGET_HAS_andc_vec         1
> 
> though this is just a bodge that (hopefully) turns the use of v128
> off entirely.
> 
> -- PMM

-- 
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