Re: [Xen-devel] mini-os: x86_64: crash passing double arguments

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2014-07-02 at 13:01 +0100, Jan Beulich wrote:
> >>> On 02.07.14 at 13:39, <Ian.Campbell@xxxxxxxxxx> wrote:
> > On Wed, 2014-07-02 at 12:32 +0100, Jan Beulich wrote:
> >> >>> On 02.07.14 at 13:16, <talex5@xxxxxxxxx> wrote:
> >> > On 2 July 2014 11:36, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> >> >>>>> On 02.07.14 at 12:17, <talex5@xxxxxxxxx> wrote:
> >> >>> Subtracting 8 bytes from thread->sp when creating a new thread fixes
> >> >>> it (patch attached), but I'm not sure whether this is the right
> >> >>> solution, or whether there's a problem elsewhere (I know very little
> >> >>> about x86).
> >> >>
> >> >> Considering that this really is kernel code, passing
> >> >> -mpreferred-stack-boundary=2 to gcc would seem like the better
> >> >> option to me, or else someone might run into a similar issue again
> >> >> trying to use %ymm or %zmm registers in their MiniOS incarnation.
> >> > 
> >> > Could you explain this a bit further? It looks like using
> >> > -mpreferred-stack-boundary=2 just means that the code won't compile
> >> > ("error: -mpreferred-stack-boundary=2 is not between 4 and 12").
> >> 
> >> Yeah, I should have written "3" (2 is for ix86). Linux uses 3, so it
> >> certainly works (but without checking I can't immediately say
> >> whether that's perhaps tied to the use of -mcmodel=kernel).
> >> 
> >> > Although Mini-OS applications run in kernel mode, they can still be
> >> > full applications, and need to use all the processor's features.
> >> 
> >> Right, and that option doesn't preclude that use. It's just that
> >> the compiler will take care to align the stack suitably in each
> >> function when it's told that the to be expected alignment is less
> >> strict than the one needed, or to use instructions not requiring
> >> full alignment.
> > 
> > Wouldn't it be more efficient to simply arrange that the thread's
> > initial stack meets the necessary preconditions for the standard
> > alignments like Thomas has done though? Especially given the trivial
> > nature of the patch.
> 
> But that can't be done once and forever, as pointed out before: If
> in the future someone wants his MiniOS incarnation to use AVX or
> AVX512, the now enforced 16-byte alignment would still not be
> enough, it would need growing to 32 or 64 bytes. By using the
> named command line option one can make the compiler take care
> of this.
> 
> Of course an option would be to use his patch to ensure 16-byte
> alignment and -mpreferred-stack-boundary=4 to cover eventual
> use of the wider registers.

I was assuming that 16-bytes (-mpreferred-stack-boundary=4) was the
implied default by the architecture's ABI, hence the compiler would
already naturally deal with things requiring 32 or 64 byte alignment
(with a starting assumption of 16-byte alignment on function entry).

If that's not the case then we should certainly make it explicit on way
or another.

Ian,


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