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

[Peter Maydell]

On Mon, 25 Feb 2019 at 13:06, Peter Maydell <address@hidden> wrote:
>
> On Mon, 25 Feb 2019 at 12:22, Natanael Copa <address@hidden> wrote:
> >
> > On Mon, 25 Feb 2019 10:34:23 +0000
> > Peter Maydell <address@hidden> wrote:
> > > The short term fix is to fix your toolchain/compilation
> > > environment options so that it isn't trying to override
> > > the definition of memcpy().
> >
> > The easiest workaround is to simply disable FORTIY_SOURCE, but that
> > will weaken the security for all implemented string functions, strcpy,
> > memmove etc, so I don't want to do that.
> >
> > Is it only lduw_he_p that needs to be atomic or are the other functions
> > in include/qemu/bswap.h using memcpy also required to be atomic?
>
> Hard to say, since we haven't done the "audit all the callers"
> step that Stefan mentioned. If you're going to replace memcpy
> with __builtin_memcpy then the safest thing is to do it for
> all those uses (this will also give you much better generated
> code for performance purposes).

I mentioned this to Richard on IRC the other day, but I think there's
a good argument for making upstream QEMU use __builtin_memcpy in
these helpers:
 * it makes us robust against things like this fortify library
   in the short term (until we fix QEMU to have an equivalent set
   of functions for doing atomic accesses to AddressSpaces)
 * in the longer term it will mean that we don't end up with
   these functions being really badly-performing even if the
   semantics of the out-of-line memcpy() are correct

thanks
-- PMM