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Re: [Qemu-devel] [Qemu-block] Guest unresponsive after Virtqueue size ex

From: Peter Maydell
Subject: Re: [Qemu-devel] [Qemu-block] Guest unresponsive after Virtqueue size exceeded error
Date: Mon, 25 Feb 2019 10:34:23 +0000

On Mon, 25 Feb 2019 at 10:24, Natanael Copa <address@hidden> wrote:
> On Sat, 23 Feb 2019 16:18:15 +0000
> Peter Maydell <address@hidden> wrote:
> > On Sat, 23 Feb 2019 at 16:05, Natanael Copa <address@hidden> wrote:
> > > I was thinking of something in the lines of:
> > >
> > > typedef volatile uint16_t __attribute__((__may_alias__)) 
> > > volatile_uint16_t;
> > > static inline int lduw_he_p(const void *ptr)
> > > {
> > >      volatile_uint16_t r = *(volatile_uint16_t*)ptr;
> > >      return r;
> > > }
> >
> > This won't correctly handle accesses with unaligned pointers,
> > I'm afraid. We rely on these functions correctly working
> > with pointers that are potentially unaligned.
> Well, current code does not even handle access with aligned pointers,
> depending on FORTIFY_SOURCE implementation.

It correctly handles aligned and unaligned pointers for the
API guarantees that the function in QEMU provides, which is
to say "definitely works on any kind of aligned or unaligned
pointer, not guaranteed to be atomic". Unfortunately some
code in QEMU is implicitly assuming it is atomic, which this
QEMU function does not guarantee -- it just happens to provide
that most of the time.

> My thinking here is that we depend on assumption that compiler will
> remove the memcpy call, so maybe find other way to generate same
> assembly, while still depend on compiler optimization.

> I did some tests and compared the assembly output. The compiler will
> generate same assembly if volatile is not used. The attribute
> __may_alias__ does not seem to make any difference. So the following
> function generates exactly same assembly:
> static inline int lduw_he_p(const void *ptr)
> {
>         uint16_t r = *(uint16_t*)ptr;
>         return r;
> }

This still is not guaranteed to work on unaligned pointers.
(For instance it probably won't work on SPARC hosts.)
More generally there is no way to have a single function
that is guaranteed to handle unaligned pointers and also
guaranteed to produce an atomic access on all host architectures
that we support, because not all host architectures allow
you to do both with the same instruction sequence. So
you can't just change this function to make it provide
atomic access without breaking the other guarantee it is

The long term fix for this is that we need to separate out
our APIs so we can have a family of functions that guarantee
to work on unaligned pointers, and a family that guarantee
to work atomically, and calling code can use the one that
provides the semantics it requires.

The short term fix is to fix your toolchain/compilation
environment options so that it isn't trying to override
the definition of memcpy().

-- PMM

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