linux-kernel - Re: [PATCHv4 2/2] iommu/vt-d: Limit number of faults to clear in irq handler

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Date:   Thu, 3 May 2018 10:16:29 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org,
        joro@...tes.org, "Raj, Ashok" <ashok.raj@...el.com>
Cc:     0x7f454c46@...il.com, Alex Williamson <alex.williamson@...hat.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        iommu@...ts.linux-foundation.org
Subject: Re: [PATCHv4 2/2] iommu/vt-d: Limit number of faults to clear in irq
 handler

Hi,

On 05/03/2018 09:59 AM, Dmitry Safonov wrote:
> On Thu, 2018-05-03 at 09:32 +0800, Lu Baolu wrote:
>> Hi,
>>
>> On 05/03/2018 08:52 AM, Dmitry Safonov wrote:
>>> AFAICS, we're doing fault-clearing in a loop inside irq handler.
>>> That means that while we're clearing if a fault raises, it'll make
>>> an irq level triggered (or on edge) on lapic. So, whenever we
>>> return
>>> from the irq handler, irq will raise again.
>>>
>> Uhm, double checked with the spec. Interrupts should be generated
>> since we always clear the fault overflow bit.
>>
>> Anyway, we can't clear faults in a limited loop, as the spec says in
>> 7.3.1:
> Mind to elaborate?
> ITOW, I do not see a contradiction. We're still clearing faults in FIFO
> fashion. There is no limitation to do some spare work in between
> clearings (return from interrupt, then fault again and continue).

Hardware maintains an internal index to reference the fault recording
register in which the next fault can be recorded. When a fault comes,
hardware will check the Fault bit (bit 31 of the 4th 32-bit register recording
register) referenced by the internal index. If this bit is set, hardware will
not record the fault.

Since we now don't clear the F bit until a register entry which has the F bit
cleared, we might exit the fault handling with some register entries still
have the F bit set.

  F
| 0 |  xxxxxxxxxxxxx|
| 0 |  xxxxxxxxxxxxx|
| 0 |  xxxxxxxxxxxxx|  <--- Fault record index in fault status register
| 0 |  xxxxxxxxxxxxx|
| 1 |  xxxxxxxxxxxxx|  <--- hardware maintained index
| 1 |  xxxxxxxxxxxxx|
| 1 |  xxxxxxxxxxxxx|
| 0 |  xxxxxxxxxxxxx|
| 0 |  xxxxxxxxxxxxx|
| 0 |  xxxxxxxxxxxxx|
| 0 |  xxxxxxxxxxxxx|

Take an example as above, hardware could only record 2 more faults with
others all dropped.

Best regards,
Lu Baolu