Re: [PATCH] [RFC] vpci: allow BAR write while mapped

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.03.2025 18:48, Roger Pau Monné wrote:
> On Wed, Mar 12, 2025 at 03:50:17PM -0400, Stewart Hildebrand wrote:
>> @@ -585,10 +601,31 @@ static void cf_check bar_write(
>>      {
>>          /* If the value written is the current one avoid printing a 
>> warning. */
>>          if ( val != (uint32_t)(bar->addr >> (hi ? 32 : 0)) )
>> +        {
>>              gprintk(XENLOG_WARNING,
>> -                    "%pp: ignored BAR %zu write while mapped\n",
>> +                    "%pp: allowing BAR %zu write while mapped\n",
>>                      &pdev->sbdf, bar - pdev->vpci->header.bars + hi);
> 
> If Xen now handles BARs writes with memory decoding enabled the
> message can be removed.  It's only purpose was to signal this missing
> handling.
> 
>> -        return;
>> +            ASSERT(rw_is_write_locked(&pdev->domain->pci_lock));
>> +            ASSERT(spin_is_locked(&pdev->vpci->lock));
>> +            reenable = true;
>> +            cmd = pci_conf_read16(pdev->sbdf, PCI_COMMAND);
>> +            /*
>> +             * Write-while-mapped: unmap the old BAR in p2m. We want this to
>> +             * finish right away since the deferral machinery only supports
>> +             * unmap OR map, not unmap-then-remap. Ultimately, it probably 
>> would
>> +             * be better to defer the write-while-mapped case just like 
>> regular
>> +             * BAR writes (but still only allow it for 32-bit BAR writes).
>> +             */
>> +            /* Disable memory decoding */
>> +            modify_bars(pdev, cmd & ~PCI_COMMAND_MEMORY, false);
> 
> I think if the guest explicitly avoids toggling memory decoding Xen
> should also to the same, and not touch the bit.

For Dom0 I'm inclined to agree, but for DomU-s it may be unsafe to do so.
(You may have meant it like this, but you said "guest".)

Jan