Re: [PATCH 11/11] xen/arm: Process pending vPCI map/unmap operations

[Julien Grall <julien@xen.org>]

Hi Oleksandr,

On 06/09/2021 10:14, Oleksandr Andrushchenko wrote:
> 
> On 06.09.21 11:48, Julien Grall wrote:
>> On 06/09/2021 08:02, Oleksandr Andrushchenko wrote:
>>> Hi, Julien!
>>
>> Hi Oleksandr,
>>
>>> On 03.09.21 12:04, Julien Grall wrote:
>>>> Hi Oleksandr,
>>>>
>>>> On 03/09/2021 09:33, Oleksandr Andrushchenko wrote:
>>>>> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
>>>>>
>>>>> vPCI may map and unmap PCI device memory (BARs) being passed through which
>>>>> may take a lot of time. For this those operations may be deferred to be
>>>>> performed later, so that they can be safely preempted.
>>>>> Run the corresponding vPCI code while switching a vCPU.
>>>>
>>>> IIUC, you are talking about the function map_range() in xen/drivers/vpci/header. The function has the following todo for Arm:
>>>>
>>>>           /*
>>>>            * ARM TODOs:
>>>>            * - On ARM whether the memory is prefetchable or not should be passed
>>>>            *   to map_mmio_regions in order to decide which memory attributes
>>>>            *   should be used.
>>>>            *
>>>>            * - {un}map_mmio_regions doesn't support preemption.
>>>>            */
>>>>
>>>> This doesn't seem to be addressed in the two series for PCI passthrough sent so far. Do you have any plan to handle it?
>>>
>>> No plan yet.
>>>
>>> All the mappings are happening with p2m_mmio_direct_dev as of now.
>>
>> So this address the first TODO but how about the second TODO? It refers to the lack of preemption on Arm but in this patch you suggest there are some and hence we need to call vpci_process_pending().
>>
>> For a tech preview, the lack of preemption would be OK. However, the commit message should be updated to make clear there are no such preemption yet or avoid to mention it.
> 
> Well, the comment was not added by me (by Roger I guess), I just keep it.

I don't think it matters to know who added it. What matters is when 
those comments are going to be handled. If they are already handled, 
then they should be dropped.

If they are not, the two TODOs listed above are probably OK to defer as 
you only plan a tech preview. But they would need to be handled before 
vCPI is selected by default and used in production.

Note that I specifically wrote "the two TODOs listed above" because I 
haven't looked at the other TODOs/FIXMEs and figued out they are fine to 
defer.

> 
> As to the preemption both map and unmap are happening via vpci_process_pending, so

Right... this doesn't mean preemption is actually supported on Arm. 
vpci_process_pending() doesn't do the preemption itself. It relies on 
map_range() to do it.

But even map_range() relies on the arch specific helper 
{,un}map_mmio_regions() to do it. If you look at the x86 implementation 
they are adding at max MAX_MMIO_MAX_ITER entries per call. On Arm, there 
are not such limit. Therefore the function will always do the full 
{,un}mapping before returning. IOW there are no preemption supported.

> 
> what is true for map is also true for unmap with this respect
> 
>>
>>>
>>>>
>>>>>
>>>>> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
>>>>> ---
>>>>>     xen/arch/arm/traps.c | 6 ++++++
>>>>>     1 file changed, 6 insertions(+)
>>>>>
>>>>> diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
>>>>> index 219ab3c3fbde..1571fb8afd03 100644
>>>>> --- a/xen/arch/arm/traps.c
>>>>> +++ b/xen/arch/arm/traps.c
>>>>> @@ -34,6 +34,7 @@
>>>>>     #include <xen/symbols.h>
>>>>>     #include <xen/version.h>
>>>>>     #include <xen/virtual_region.h>
>>>>> +#include <xen/vpci.h>
>>>>>       #include <public/sched.h>
>>>>>     #include <public/xen.h>
>>>>> @@ -2304,6 +2305,11 @@ static bool check_for_vcpu_work(void)
>>>>>         }
>>>>>     #endif
>>>>>     +    local_irq_enable();
>>>>> +    if ( has_vpci(v->domain) && vpci_process_pending(v) )
>>>>
>>>> Looking at the code of vpci_process_pending(), it looks like there are some rework to do for guest. Do you plan to handle it as part of the vPCI series?
>>> Yes, vPCI code is heavily touched to support guest non-identity mappings
>>
>> I wasn't referring to the non-identity mappings here. I was referring to TODOs such as:
>>
>>              /*
>>               * FIXME: in case of failure remove the device from the domain.
>>               * Note that there might still be leftover mappings. While this is
>>               * safe for Dom0, for DomUs the domain will likely need to be
>>               * killed in order to avoid leaking stale p2m mappings on
>>               * failure.
>>               */
>>
>> You still have them after the series reworking the vPCI. As for the preemption this is OK to ignore it for a tech preview. Although, we want to at least track them.
> Please see above: both map and unmap are happening via vpci_process_pending

I am not sure how this is relevant to what I just mentionned.

>>
>>>>
>>>>> + raise_softirq(SCHEDULE_SOFTIRQ);
>>>>> +    local_irq_disable();
>>>>> +
>>>>
>>>>   From my understanding of vcpi_process_pending(). The function will return true if there are more work to schedule.
>>> Yes
>>>> However, if check_for_vcpu_for_work() return false, then we will return to the guest before any work for vCPI has finished. This is because check_for_vcpu_work() will not be called again.
>>> Correct
>>>>
>>>> In this case, I think you want to return as soon as you know we need to reschedule.
>>> Not sure I understand this
>>
> I was more referring to "I think you want to return as soon as you know we need to reschedule."
>> The return value of check_for_vcpu_for_work() indicates whether we have more work to do before returning to return to the guest.
>>
>> When vpci_process_pending() returns true, it tells us we need to call the function at least one more time before returning to the guest.
>>
>> In your current implementation, you leave that decision to whoeever is next in the function.
>>
>> It is not safe to return to the guest as long as vpci_process_pending() returns true. So you want to write something like:
>>
>> if ( vpci_process_pending() )
>>    return true;
> --- a/xen/arch/arm/traps.c
> 
> +++ b/xen/arch/arm/traps.c
> @@ -2291,6 +2291,9 @@ static bool check_for_vcpu_work(void)
>    {
>        struct vcpu *v = current;
> 
> +    if ( vpci_process_pending() )
> +      return true;
> +
>    #ifdef CONFIG_IOREQ_SERVER
>        if ( domain_has_ioreq_server(v->domain) )
>        {
> Do you mean something like this?

Yes.

>>>> However, looking at the rest of the code, we already have a check for vpci in the common IOREQ code.
>>>
>>> Which may not be enabled as it depends on CONFIG_IOREQ_SERVER.
>>
>> Right. My point is when CONFIG_IOREQ_SERVER is set then you would end up to call twice vpci_process_pending(). This will have an impact how on long your vCPU is going to running because you are doubling the work.
> 
> So, you suggest that we have in the common IOREQ code something call like
> 
> arch_vpci_process_pending? In case of x86 it will have the code currently found in the
> 
> common IOREQ sources and for Arm it will be nop?

No I am suggesting to move the call of the IOREQ code to hvm_do_resume() 
(on x86) and check_for_vcpu_work() (on Arm).

Cheers,

-- 
Julien Grall