On 14.03.2025 09:21, Roger Pau Monné wrote:
> On Fri, Mar 14, 2025 at 09:04:55AM +0100, Jan Beulich wrote:
>> 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".)
>
> Sorry, I'm not sure I'm following. For domUs the BAR register write
> is not propagated to the hardware, it's just the p2m mappings that
> change, and hence it's even safer to not toggle the memory decoding
> bit for that case? (as there's no write to the device BAR registers
> for domUs).
Oh, right. I'm sorry for the noise.
Jan
