Re: [PATCH v3 5/7] vpci: add SR-IOV support for PVH Dom0

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.04.2026 12:12, Mykyta Poturai wrote:
> On 4/21/26 17:43, Jan Beulich wrote:
>> On 09.04.2026 16:01, Mykyta Poturai wrote:
>>> From: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
>>>
>>> This code is expected to only be used by privileged domains,
>>> unprivileged domains should not get access to the SR-IOV capability.
>>>
>>> Implement RW handlers for PCI_SRIOV_CTRL register to dynamically
>>> map/unmap VF BARS. Recalculate BAR sizes before mapping VFs to account
>>> for possible changes in the system page size register. Also force VFs to
>>> always use emulated reads for command register, this is needed to
>>> prevent some drivers accidentally unmapping BARs.
>>
>> This apparently refers to the change to vpci_init_header(). Writes are
>> already intercepted. How would a read lead to accidental BAR unmap? Even
>> for writes I don't see how a VF driver could accidentally unmap BARs, as
>> the memory decode bit there is hardwired to 0.
>>
>>> Discovery of VFs is
>>> done by Dom0, which must register them with Xen.
>>
>> If we intercept control register writes, why would we still require
>> Dom0 to report the VFs that appear?
>>
> 
> Sorry, I don't understand this question. You specifically requested this 
> to be done this way in V2. Quoting your reply from V2 below.
> 
>  > Aren't you effectively busy-waiting for these 100ms, by simply 
> returning "true"
>  > from vpci_process_pending() until the time has passed? This imo is a 
> no-go. You
>  > want to set a timer and put the vCPU to sleep, to wake it up again 
> when the
>  > timer has expired. That'll then eliminate the need for the 
> not-so-nice patch 4.
> 
>  > Question is whether we need to actually go this far (right away). I 
> expect you
>  > don't mean to hand PFs to DomU-s. As long as we keep them in the hardware
>  > domain, can't we trust it to set things up correctly, just like we 
> trust it in
>  > a number of other aspects?

How's any of this related to the question I raised here, or your reply
thereto? If we intercept PCI_SRIOV_CTRL, we know when VFs are created.
Why still demand Dom0 to report them then?

>>> +static int map_vfs(const struct pci_dev *pf_pdev, uint16_t cmd)
>>> +{
>>> +    struct pci_dev *vf_pdev;
>>> +    int rc;
>>> +
>>> +    ASSERT(rw_is_write_locked(&pf_pdev->domain->pci_lock));
>>> +
>>> +    list_for_each_entry(vf_pdev, &pf_pdev->vf_list, vf_list)
>>> +    {
>>> +        rc = vpci_modify_bars(vf_pdev, cmd, false);
>>> +        if ( rc )
>>> +        {
>>> +            gprintk(XENLOG_ERR, "failed to %s VF %pp: %d\n",
>>> +                    (cmd & PCI_COMMAND_MEMORY) ? "map" : "unmap",
>>> +                    &vf_pdev->sbdf, rc);
>>> +            return rc;
>>> +        }
>>> +
>>> +        vf_pdev->vpci->header.guest_cmd &= ~PCI_COMMAND_MEMORY;
>>> +        vf_pdev->vpci->header.guest_cmd |= (cmd & PCI_COMMAND_MEMORY);
>>
>> As mentioned elsewhere as well, this bit is supposed to be 0 for VFs.
> 
> There are some devices that expose VFs with the same VID/DID as in the 
> PF, causing Linux to use normal driver for them and threat them like 
> normal devices. At some point, those normal drivers try to do a 
> read-modify-update of the command register and end up writing 0 to 
> PCI_COMMAND_MEMORY, causing cmd_write to unmap the BARS of that device. 
> I am not sure, maybe it would be better to just ignore cmd writes for VFs?

No. We should treat r/o bits as r/o (which for this bit implies it not
controlling BAR mapping).

>>> +    sriov_pos = pci_find_ext_capability(pf_pdev, PCI_EXT_CAP_ID_SRIOV);
>>> +    ctrl = pci_conf_read16(pf_pdev->sbdf, sriov_pos + PCI_SRIOV_CTRL);
>>> +
>>> +    if ( (pf_pdev->domain == vf_pdev->domain) && (ctrl & 
>>> PCI_SRIOV_CTRL_MSE) )
>>> +    {
>>> +        rc = vpci_modify_bars(vf_pdev, PCI_COMMAND_MEMORY, false);
>>
>> Doesn't VF enable also need to be set for the BARs to be mapped?
> 
> I don't think so. Enabling memory space logically maps very well to 
> mapping memory to the guest. I don’t see any benefit of also requiring 
> VFE bit here.

Iirc the spec is quite explicit in this regard.

Jan