Re: [Qemu-devel] [RFC Patch] xen/pt: Emulate FLR capability

[Chao Gao]

On Thu, Aug 29, 2019 at 12:03:44PM +0200, Jan Beulich wrote:
>On 29.08.2019 11:02, Chao Gao wrote:
>> Currently, for a HVM on Xen, no reset method is virtualized. So in a VM's
>> perspective, assigned devices cannot be reset. But some devices rely on PCI
>> reset to recover from hardware hangs. When being assigned to a VM, those
>> devices cannot be reset and won't work any longer if a hardware hang occurs.
>> We have to reboot VM to trigger PCI reset on host to recover the device.
>
>Did you consider a hot-unplug, reset (by host), hot-plug cycle instead?

Yes. I considered this means. But it needs host to initiate this action.
However, when a device needs reset is determined by the device driver
in VM. So in practice, VM still needs a way to notify host to do
unplug/reset/plug. As the standard FLR capability can meet the
requirement, I don't try to invent one.

>
>> +static int xen_pt_devctl_reg_write(XenPCIPassthroughState *s,
>> +                                   XenPTReg *cfg_entry, uint16_t *val,
>> +                                   uint16_t dev_value, uint16_t valid_mask)
>> +{
>> +    if (s->real_device.is_resetable && (*val & PCI_EXP_DEVCTL_BCR_FLR)) {
>> +        xen_pt_reset(s);
>> +    }
>> +    return xen_pt_word_reg_write(s, cfg_entry, val, dev_value, valid_mask);
>
>I think you also need to clear the bit before handing on the request,
>such that reads will always observe it clear.

Will do.

Thanks
Chao