Re: [Qemu-devel] [PATCH 4/4] kvm: i386: Add classic PCI device assignment

[Blue Swirl]

On Tue, Aug 28, 2012 at 7:31 PM, Anthony Liguori <address@hidden> wrote:
> Blue Swirl <address@hidden> writes:
>
>> On Tue, Aug 28, 2012 at 5:28 PM, Michael S. Tsirkin <address@hidden> wrote:
>>> On Tue, Aug 28, 2012 at 05:01:55PM +0000, Blue Swirl wrote:
>>>> On Tue, Aug 28, 2012 at 7:35 AM, Michael Tokarev <address@hidden> wrote:
>>>> > On 27.08.2012 22:56, Blue Swirl wrote:
>>>> > []
>>>> >>> +static uint32_t slow_bar_readb(void *opaque, target_phys_addr_t addr)
>>>> >>> +{
>>>> >>> +    AssignedDevRegion *d = opaque;
>>>> >>> +    uint8_t *in = d->u.r_virtbase + addr;
>>>> >>
>>>> >> Don't perform arithmetic with void pointers.
>>>> >
>>>> > There are a few places in common qemu code which does this for a very
>>>> > long time.  So I guess it is safe now.
>>>>
>>>> It's a non-standard GCC extension.
>>>
>>> So?  We use many other GCC extensions. grep for typeof.
>>
>> Dependencies should not be introduced trivially. In this case, it's
>> pretty easy to avoid void pointer arithmetic as Jan's next version
>> shows.
>
> The standard is vague with respect void arithmetic.  Most compilers
> allow it.  A very good analysis of the standard can be found below.
>
> http://stackoverflow.com/questions/3523145/pointer-arithmetic-for-void-pointer-in-c

The analysis would seem to show that arithmetic may be acceptable, but
it doesn't say that void pointers must be treated like char pointers.
In my view, this would make sense:

char *cptr;
void *vptr;

Since
cptr++;
is equivalent to
cptr = (char *)((uintptr_t)cptr + sizeof(*cptr));

therefore

vptr++;
should be equivalent to
vptr = (void *)((uintptr_t)vptr + sizeof(*vptr));

That is, vptr++ should be equivalent to vptr += 0 because sizeof(void)
should be 0 if allowed.

>
> BTW: can we please stop arguing about C standards.  If we currently are
> using something in QEMU that's supported by clang and GCC, it's fine and
> we ought to continue using it.
>
> The reserved names actually did bite us when porting to a new platform.
> But the only requirement for C extensions ought to be reasonable support
> in GCC and clang.
>
> I don't care at all about supporting proprietary compilers.

We also don't have crowds banging doors with their money bags with a
need for such support.

>
> Regards,
>
> Anthony Liguori
>
>>
>>>
>>> Is there a work in progress to build GCC with visual studio?
>>> If yes what are the chances KVM device assignment
>>> will work on windows?
>>
>> IIRC there was really a project to use KVM on Windows and another
>> project to build QEMU with MSVC.
>>
>>>
>>> Look QEMU codebase is what it is. Unless you rework all existing
>>> code to confirm to your taste, I do not see why you NACK valid new code
>>> unless it confirms to same.
>>
>> Yes, I'd be happy to fix the style with huge patches at once. But our
>> fearless leader does not agree, so we are stuck with the codebase
>> being what it is until it is fixed one step at a time.
>>
>>>
>>>> >
>>>> > /mjt
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