Re: Design session "MSI-X support with Linux stubdomain" notes

[Juergen Gross <jgross@xxxxxxxx>]

On 29.09.22 13:52, Roger Pau Monné wrote:
> On Thu, Sep 29, 2022 at 01:44:28PM +0200, Jan Beulich wrote:
> > On 29.09.2022 12:57, Marek Marczykowski-Górecki wrote:
> > > On Mon, Sep 26, 2022 at 02:47:55PM +0200, Jan Beulich wrote:
> > > > On 26.09.2022 14:43, Marek Marczykowski-Górecki wrote:
> > > > > On Thu, Sep 22, 2022 at 08:00:00PM +0200, Jan Beulich wrote:
> > > > > > On 22.09.2022 18:05, Anthony PERARD wrote:
> > > > > > > WARNING: Notes missing at the beginning of the meeting.
> > > > > > >
> > > > > > > session description:
> > > > > > > > Currently a HVM with PCI passthrough and Qemu Linux stubdomain doesn’t
> > > > > > > > support MSI-X. For the device to (partially) work, Qemu needs a patch masking
> > > > > > > > MSI-X from the PCI config space. Some drivers are not happy about that, which
> > > > > > > > is understandable (device natively supports MSI-X, so fallback path are
> > > > > > > > rarely tested).
> > > > > > > >
> > > > > > > > This is mostly (?) about qemu accessing /dev/mem directly (here:
> > > > > > > > https://github.com/qemu/qemu/blob/master/hw/xen/xen_pt_msi.c#L579) - lets
> > > > > > > > discuss alternative interface that stubdomain could use.
> > > > > > >
> > > > > > >
> > > > > > > when qemu forward interrupt,
> > > > > > >      for correct mask bit, it read physical mask bit.
> > > > > > >      an hypercall would make sense.
> > > > > > >      -> benefit, mask bit in hardware will be what hypervisor desire, and 
> > > > > > > device model desire.
> > > > > > >      from guest point of view, interrupt should be unmask.
> > > > > > >
> > > > > > > interrupt request are first forwarded to qemu, so xen have to do some post 
> > > > > > > processing once request comes back from qemu.
> > > > > > >      it's weird..
> > > > > > >
> > > > > > > someone should have a look, and rationalize this weird path.
> > > > > > >
> > > > > > > Xen tries to not forward everything to qemu.
> > > > > > >
> > > > > > > why don't we do that in xen.
> > > > > > >      there's already code in xen for that.
> > > > > > So what I didn't pay enough attention to when talking was that the
> > > > > > completion logic in Xen is for writes only. Maybe something similar
> > > > > > can be had for reads as well, but that's to be checked ...
> > > > > I spent some time trying to follow that part of qemu, and I think it
> > > > > reads vector control only on the write path, to keep some bits
> > > > > unchanged, and also detect whether Xen masked it behind qemu's back.
> > > > > My understanding is, since 484d7c852e "x86/MSI-X: track host and guest
> > > > > mask-all requests separately" it is unnecessary, because Xen will
> > > > > remember guest's intention, so qemu can simply use its own internal
> > > > > state and act on that (guest writes will go through qemu, so it should
> > > > > have up to date view from guest's point of view).
> > > > >
> > > > > As for PBA access, it is read by qemu only to pass it to the guest. I'm
> > > > > not sure whether qemu should use hypercall to retrieve it, or maybe
> > > > > Xen should fixup value itself on the read path.
> > > > Forwarding the access to qemu just for qemu to use a hypercall to obtain
> > > > the value needed seems backwards to me. If we need new code in Xen, we
> > > > can as well handle the read directly I think, without involving qemu.
> > > I'm not sure if I fully follow what qemu does here, but I think the
> > > reason for such handling is that PBA can (and often do) live on the same
> > > page as the actual MSI-X table. I'm trying to adjust qemu to not
> > > intercept this read, but at this point I'm not yet sure of that's even
> > > possible on sub-page granularity.
> > >
> > > But, to go forward with PoC/debugging, I hardwired PBA read to
> > > 0xFFFFFFFF, and it seems it doesn't work. My observation is that the
> > > handler in the Linux driver isn't called. There are several moving
> > > part (it could very well be bug in the driver, or some other part in the
> > > VM). Is there some place in Xen I can see if an interrupt gets delivered
> > > to the guest (some function I can add debug print to), or is it
> > > delivered directly to the guest?
> > I guess "iommu=no-intpost" would suppress "direct" delivery (if hardware
> > is capable of that in the first place). And wait - this option actually
> > default to off.
> >
> > As to software delivery - I guess you would want to start from
> > do_IRQ_guest() and then see where things get lost. (Adding logging to
> > such a path of course has a fair risk of ending up overly chatty.)
> Having dealt with interrupt issues before, try to limit logging to the
> IRQ you are interested on only - using xentrace might be a better
> option depending on what you need to debug, albeit it's kind of a pain
> to add new trace points as you also need to modify xenalyze to print
> them.
Did you consider using debugtrace_printk()?


Juergen

Attachment: OpenPGP_0xB0DE9DD628BF132F.asc
Description: OpenPGP public key

Attachment: OpenPGP_signature
Description: OpenPGP digital signature