Re: [PATCH 2/2] iommu/vt-d: don's issue devTLB flush request when device is disconnected

Ethan Zhao
2023-12-15T00:43:30Z

On 12/14/2023 10:16 AM, Ethan Zhao wrote:
>
> On 12/13/2023 6:44 PM, Lukas Wunner wrote:
>> On Tue, Dec 12, 2023 at 10:46:37PM -0500, Ethan Zhao wrote:
>>> For those endpoint devices connect to system via hotplug capable ports,
>>> users could request a warm reset to the device by flapping device's
>>> link
>>> through setting the slot's link control register,
>> Well, users could just *unplug* the device, right?  Why is it relevant
>> that thay could fiddle with registers in config space?
>>
> Yes, if the device and it's slot are hotplug capable, users could just
>
> 'unplug' the device.
>
> But this case reported, users try to do a warm reset with a tool
>
> command like:
>
>   mlxfwreset -d <busid> -y reset
>
> Actually, it will access configuration space  just as
>
>  setpci -s 0000:17:01.0 0x78.L=0x21050010
>
> Well, we couldn't say don't fiddle PCIe config space registers like
>
> that.
>
>>> as pciehpt_ist() DLLSC
>>> interrupt sequence response, pciehp will unload the device driver and
>>> then power it off. thus cause an IOMMU devTLB flush request for
>>> device to
>>> be sent and a long time completion/timeout waiting in interrupt
>>> context.
>> A completion timeout should be on the order of usecs or msecs, why
>> does it
>> cause a hard lockup?  The dmesg excerpt you've provided shows a 12
>> *second*
>> delay between hot removal and watchdog reaction.
>>
> In my understanding, the devTLB flush request sent to ATS capable devcie
>
> is non-posted request, if the ATS transaction is broken by endpoint link
>
> -down, power-off event, the timeout will take up to 60 seconds+-30,
>
> see "Invalidate Completion Timeout " part of
>
> chapter 10.3.1 Invalidate Request
>
> In PCIe spec 6.1
>
> "
>
> IMPLEMENTATION NOTE:
>
> INVALIDATE COMPLETION TIMEOUT
>
> Devices should respond to Invalidate Requests within 1 minute (+50%
> -0%).Having a bounded time
>
> permits an ATPT to implement Invalidate Completion Timeouts and reuse
> the associated ITag values.
>
> ATPT designs are implementation specific. As such, Invalidate
> Completion Timeouts and their
>
> associated error handling are outside the scope of this specification
>
> "
>
>>> Fix it by checking the device's error_state in
>>> devtlb_invalidation_with_pasid() to avoid sending meaningless devTLB
>>> flush
>>> request to link down device that is set to
>>> pci_channel_io_perm_failure and
>>> then powered off in
>> This doesn't seem to be a proper fix.  It will work most of the time
>> but not always.  A user might bring down the slot via sysfs, then yank
>> the card from the slot just when the iommu flush occurs such that the
>> pci_dev_is_disconnected(pdev) check returns false but the card is
>> physically gone immediately afterwards.  In other words, you've shrunk
>> the time window during which the issue may occur, but haven't eliminated
>> it completely.
>
> If you mean disable the slot via sysfs, that's SAFE_REMOVAL, right ?
>
> that would issse devTLB invalidation first, power off device later, it
>
> wouldn't trigger the hard lockup, though the
>
> pci_dev_is_disconnected() return false. this fix works such case.

Could you help to point out if there are any other window to close ?

Thanks,

Ethan

>
>
> Thanks,
>
> Ethan
>
>
>
>>
>> Thanks,
>>
>> Lukas