Re: [Xen-devel] [PATCH v3 1/2] xen/pt: fix some pass-thru devices don't work across reboot

[Chao Gao <chao.gao@xxxxxxxxx>]

On Tue, Dec 18, 2018 at 08:53:46AM -0700, Jan Beulich wrote:
>>>> On 18.12.18 at 15:43, <chao.gao@xxxxxxxxx> wrote:
>> I find some pass-thru devices don't work any more across guest
>> reboot. Assigning it to another domain also meets the same issue. And
>> the only way to make it work again is un-binding and binding it to
>> pciback. Someone reported this issue one year ago [1].
>> 
>> If the device's driver doesn't disable MSI-X during shutdown or qemu is
>> killed/crashed before the domain shutdown, this domain's pirq won't be
>> unmapped. Then xen takes over this work, unmapping all pirq-s, when
>> destroying guest. But as pciback has already disabled meory decoding before
>> xen unmapping pirq, Xen has to sets the host_maskall flag and maskall bit
>> to mask a MSI rather than sets maskbit in MSI-x table. The call trace of
>> this process is:
>> 
>> ->arch_domain_destroy
>>     ->free_domain_pirqs
>>         ->unmap_domain_pirq (if pirq isn't unmapped by qemu)
>>             ->pirq_guest_force_unbind
>>                 ->__pirq_guest_unbind
>>                     ->mask_msi_irq(=desc->handler->disable())
>>                         ->the warning in msi_set_mask_bit()
>> 
>> The host_maskall bit will prevent guests from clearing the maskall bit
>> even the device is assigned to another guest later. Then guests cannot
>> receive MSIs from this device.
>> 
>> To fix this issue, a pirq is unmapped before memory decoding is disabled by
>> pciback. Specifically, when a device is detached from a guest, all 
>> established
>> mappings between pirq and msi are destroying before changing the ownership.
>> 
>> [1]: 
>> https://lists.xenproject.org/archives/html/xen-devel/2017-09/msg02520.html 
>> 
>> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
>> ---
>> Applied this patch, qemu would report the error below:
>>     [00:05.0] msi_msix_disable: Error: Unbinding of MSI-X failed. (err: 1, 
>> pirq: 302, gvec: 0xd5)
>>     [00:05.0] msi_msix_disable: Error: Unbinding of MSI-X failed. (err: 1, 
>> pirq: 301, gvec: 0xe5)
>>     [00:04.0] msi_msix_disable: Error: Unbinding of MSI-X failed. (err: 1, 
>> pirq: 359, gvec: 0x41)
>>     [00:04.0] msi_msix_disable: Error: Unbinding of MSI-X failed. (err: 1, 
>> pirq: 358, gvec: 0x51)
>> 
>> Despite of the error, guest shutdown or device hotplug finishs smoothly.
>> It seems to me that qemu tries to unbind a msi which is already unbound by
>> the code added by this patch. I am not sure whether it is acceptable to
>> leave this error there.
>
>Well, the errors mean that qemu is playing with a device that's no
>longer owned by the guest controlled by this qemu instance. At
>least with a de-privileged qemu (no idea whether this actually works
>with pass-through) that's still a mistake, and hence would need
>fixing. Whichever entity it is that invokes the de-assign of the
>device, other involved parties should be informed so that they can
>keep their hands off the device from that point onwards.
>
>The hypervisor change itself looks mostly fine, just a few minor
>comments.
>
>> --- a/xen/drivers/passthrough/pci.c
>> +++ b/xen/drivers/passthrough/pci.c
>> @@ -368,6 +368,7 @@ static struct pci_dev *alloc_pdev(struct pci_seg *pseg, 
>> u8 bus, u8 devfn)
>>              return NULL;
>>          }
>>          spin_lock_init(&msix->table_lock);
>> +        msix->warned = DOMID_INVALID;
>
>This is an arch-specific field right now; in fact the entire structure
>is arch-specific. Playing with any of its fields in common code is
>undesirable, but I guess the use of ->table_lock can be taken as
>an excuse until this code wants to eventually be used by Arm.
>(The structure requiring a lock is sufficiently generic, whereas
>the "warned" field may not be universally needed.)

I will clean up this place.

>
>> @@ -1514,6 +1515,52 @@ static int assign_device(struct domain *d, u16 seg, 
>> u8 bus, u8 devfn, u32 flag)
>>      return rc;
>>  }
>>  
>> +/*
>> + * Unmap established mappings between domain's pirq and device's MSI.
>> + * These mappings were set up by qemu/guest and are expected to be
>> + * destroyed when changing the device's ownership.
>> + */
>> +static void pci_unmap_msi(struct pci_dev *pdev)
>> +{
>> +    struct msi_desc *entry, *tmp;
>> +
>> +    ASSERT(pcidevs_locked());
>> +
>> +    if ( !pdev->domain )
>
>There are quite a few uses of pdev->domain - please consider
>using a local variable.
>
>> +        return;
>> +
>> +    spin_lock(&pdev->domain->event_lock);
>> +    list_for_each_entry_safe( entry, tmp, &pdev->msi_list, list )
>> +    {
>> +        struct pirq *info;
>> +        struct hvm_pirq_dpci *pirq_dpci;
>> +        int pirq = domain_irq_to_pirq(pdev->domain, entry->irq), pirq_orig;
>> +
>> +        pirq_orig = pirq;
>> +
>> +        if ( !pirq )
>> +            continue;
>> +
>> +        /* For forcibly unmapped pirq, lookup radix tree with absolute 
>> value */
>> +        if ( pirq < 0)
>> +            pirq = -pirq;
>> +
>> +        info = pirq_info(pdev->domain, pirq);
>
>Why not simply
>
>        info = pirq_info(pdev->domain, ABS(pirq));
>
>without any pirq_orig?

Will do.

Thanks
Chao

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