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From:   Thomas Gleixner <tglx@linutronix.de>
To:     "Tian\, Kevin" <kevin.tian@intel.com>,
        Alex Williamson <alex.williamson@redhat.com>
Cc:     Jason Gunthorpe <jgg@nvidia.com>,
        "Dey\, Megha" <megha.dey@intel.com>,
        "Raj\, Ashok" <ashok.raj@intel.com>,
        "Pan\, Jacob jun" <jacob.jun.pan@intel.com>,
        "Jiang\, Dave" <dave.jiang@intel.com>,
        "Liu\, Yi L" <yi.l.liu@intel.com>,
        "Lu\, Baolu" <baolu.lu@intel.com>,
        "Williams\, Dan J" <dan.j.williams@intel.com>,
        "Luck\, Tony" <tony.luck@intel.com>,
        "Kumar\, Sanjay K" <sanjay.k.kumar@intel.com>,
        LKML <linux-kernel@vger.kernel.org>, KVM <kvm@vger.kernel.org>,
        Kirti Wankhede <kwankhede@nvidia.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Marc Zyngier <maz@kernel.org>,
        Bjorn Helgaas <helgaas@kernel.org>
Subject: RE: Virtualizing MSI-X on IMS via VFIO
In-Reply-To: <MWHPR11MB18864420ACE88E060203F7818C079@MWHPR11MB1886.namprd11.prod.outlook.com>
References: <20210622131217.76b28f6f.alex.williamson@redhat.com> <87o8bxcuxv.ffs@nanos.tec.linutronix.de> <20210623091935.3ab3e378.alex.williamson@redhat.com> <MWHPR11MB18864420ACE88E060203F7818C079@MWHPR11MB1886.namprd11.prod.outlook.com>
Date:   Thu, 24 Jun 2021 03:36:07 +0200
Message-ID: <87r1gsavso.ffs@nanos.tec.linutronix.de>
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Kevin,

On Thu, Jun 24 2021 at 00:00, Kevin Tian wrote:
>> From: Alex Williamson <alex.williamson@redhat.com>
>> Sent: Wednesday, June 23, 2021 11:20 PM
>>
> [...]
>  > > So the only downside today of allocating more MSI-X vectors than
>> > necessary is memory consumption for the irq descriptors.
>> 
>> As above, this is a QEMU policy of essentially trying to be a good
>> citizen and allocate only what we can infer the guest is using.  What's
>> a good way for QEMU, or any userspace, to know it's running on a host
>> where vector exhaustion is not an issue?
>
> In my proposal a new command (VFIO_DEVICE_ALLOC_IRQS) is
> introduced to separate allocation from enabling. The availability
> of this command could be the indicator whether vector 
> exhaustion is not an issue now?

Your proposal still does not address the fundamental issue of a missing
feedback to the guest and you can invent a gazillion more IOCTL commands
and none of them will solve that issue. A hypercall/paravirt interface is
the only reasonable solution.

The time you are wasting to come up with non-solutions would have surely
been better spent implementing the already known and obvious proper
solution. You might be halfways done already with that.

Thanks,

        tglx