Re: [PATCH v5 08/11] xen/arm: Enable the existing x86 virtual PCI support for ARM.

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi Julien,

> On 12 Oct 2021, at 16:04, Julien Grall <julien@xxxxxxx> wrote:
> 
> On 11/10/2021 13:41, Bertrand Marquis wrote:
>> Hi Jan,
> 
> Hi Bertrand,
> 
>> As Rahul is on leave, I will answer you and make the changes needed.
>>> On 7 Oct 2021, at 14:43, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>> Independent of this - is bare metal Arm enforcing this same
>>> alignment restriction (unconditionally)? Iirc on x86 we felt we'd
>>> better synthesize misaligned accesses.
>> Unaligned IO access could be synthesise also on arm to but I would
>> rather not make such a change now without testing it (and there is
>> also a question of it making sense).
> 
> Yes it makes sense. I actually have an item in my TODO list to forbid 
> unaligned access because they should not happen on any device we currently 
> emulate.
> 
> Although, I am not aware of any issue other than the guest would shoot itself 
> in the foot if this happens.
> 
>> So if it is ok with you I will keep that check and discuss it with Rahul
>> when he is back. I will add a comment in the code to make that clear.
> 
> I am OK with it.
> 
> [...]
> 
>>> Throughout this series I haven't been able to spot where the HAS_VPCI
>>> Kconfig symbol would get selected. Hence I cannot tell whether all of
>>> this is Arm64-specific. Otherwise I wonder whether size 8 actually
>>> can occur on Arm32.
>> Dabt.size could be 3 even on ARM32 but we should not allow 64bit
>> access on mmio regions on arm32.
> 
> Hmmm... Looking at the Armv7 and Armv8 spec, ldrd/strd (64-bit read) would 
> not present a valid ISV. So I think it is not be possible to have dabt.size = 
> 3 for 32-bit domain. But I agree we probably want to harden the code.
> 
>> So I will surround this code with ifdef CONFIG_ARM_64 and add a test
>> for len > 4 to prevent this case on 32bit.
>> To be completely right we should disable this also for 32bit guests but
>> this change would be a bit more invasive.
> 
> I think the following should be sufficient:
> 
> if ( is_domain_32bit_domain() && len > 4 )
>  return ...;

With the last request from Roger to use the function implemented in 
arch/x86/hw/io.c, the function will move to vpci.h so using is_32bit_domain 
will not be possible without ifdefery CONFIG_ARM.
Also I have no access to the domain there.

So the best I can do for now would be something like:
#ifdef CONFIG_ARM_32
If (len == 8)
    return false
#endif

A 32bit guest on 64bit xen would not be checked.

Would that be ok for now ?

I could add a comment in the code to warn about the limitation.

Cheers
Bertrand

> 
> Cheers,
> 
> -- 
> Julien Grall