Re: [PATCH qemu-xen-traditional] xen/pt: allow QEMU to request MSI unmasking at bind time - Paraschiv, Andra-Irina

From: "Paraschiv, Andra-Irina" <andraprs@amazon.com>
To: Ian Jackson <ian.jackson@citrix.com>
Cc: "sstabellini@kernel.org" <sstabellini@kernel.org>,
	"Paraschiv, Andra-Irina" <andraprs@amazon.com>,
	"andrew.cooper3@citrix.com" <andrew.cooper3@citrix.com>,
	"jbeulich@suse.com" <jbeulich@suse.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	"Woodhouse, David" <dwmw@amazon.co.uk>,
	"roger.pau@citrix.com" <roger.pau@citrix.com>
Subject: Re: [PATCH qemu-xen-traditional] xen/pt: allow QEMU to request MSI unmasking at bind time
Date: Tue, 9 Oct 2018 15:23:55 +0000	[thread overview]
Message-ID: <1539098633397.3404@amazon.com> (raw)
In-Reply-To: <23484.45915.156520.714628@mariner.uk.xensource.com>

> > >Ian, any objections?
>
> Sorry for dropping this.  It's been a while!

No problem, Ian, we have this heads-up now.

>
> > >> > When a MSI interrupt is bound to a guest using
> > >> > xc_domain_update_msi_irq (XEN_DOMCTL_bind_pt_irq) the interrupt
> > >> > is left masked by default.
> > >> >
> > >> > This causes problems with guests that first configure
> > >> > interrupts and clean the per-entry MSIX table mask bit and
> > >> > afterwards enable MSIX globally. In such scenario the Xen
> > >> > internal msixtbl handlers would not detect the unmasking of
> > >> > MSIX entries because vectors are not yet registered since MSIX
> > >> > is not enabled, and vectors would be left masked.
> > >> >
> > >> > Introduce a new flag in the gflags field to signal Xen whether
> > >> > a MSI interrupt should be unmasked after being bound.
> > >> >
> > >> > This also requires to track the mask register for MSI
> > >> > interrupts, so QEMU can also notify to Xen whether the MSI
> > >> > interrupt should be bound masked or unmasked
>
> I searched my email archives but I haven't been able to find a copy of
> the actual patch.  Was it ever sent to me ?
> 
> Also, qemu-trad is quite deeply frozen.  I'd like an explanation of
> the practical impact of this bug, and the use cases where the fix is
> needed, to justify the change.  Sorry to be awkward.

That's the link to the patch for qemu-xen-traditional:
https://lists.xenproject.org/archives/html/xen-devel/2018-06/msg01248.html

(https://lists.xenproject.org/archives/html/xen-devel/2018-06/msg01373.html - added you in CC)

The issue was notified here - https://lists.xenproject.org/archives/html/xen-devel/2018-05/msg01238.html and fixes were added in xen and qemu. QEMU needs to interact with Xen to have guest interrupts masked/unmasked.

This patch is a backport from qemu.

One of the use cases where this fix is needed is: guest OS is Windows and the host has the latest stable version of xen and qemu-xen-traditional. Using this environment, setting up a PCI device (using MSI/MSI-X interrupts) will not successfully finish.

Thanks,
Andra

>
> Regards,
> Ian.

Amazon Development Center (Romania) S.R.L. registered office: 27A Sf. Lazar Street, UBC5, floor 2, Iasi, Iasi County, 700045, Romania. Registered in Romania. Registration number J22/2621/2005.

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