linux-kernel - Re: [PATCH 6/9] virtio_pci: harden MSI-X interrupts

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Date:   Tue, 14 Sep 2021 00:31:36 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Hetzelt, Felicitas" <f.hetzelt@...berlin.de>,
        "kaplan, david" <david.kaplan@....com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        pbonzini <pbonzini@...hat.com>, Andi Kleen <ak@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Richard Henderson <rth@...ddle.net>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        James E J Bottomley <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>,
        "David S . Miller" <davem@...emloft.net>,
        Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Peter H Anvin <hpa@...or.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        X86 ML <x86@...nel.org>
Subject: Re: [PATCH 6/9] virtio_pci: harden MSI-X interrupts

On Mon, Sep 13 2021 at 16:54, Michael S. Tsirkin wrote:

> On Mon, Sep 13, 2021 at 09:38:30PM +0200, Thomas Gleixner wrote:
>> On Mon, Sep 13 2021 at 15:07, Jason Wang wrote:
>> > On Mon, Sep 13, 2021 at 2:50 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>> >> > But doen't "irq is disabled" basically mean "we told the hypervisor
>> >> > to disable the irq"?  What extractly prevents hypervisor from
>> >> > sending the irq even if guest thinks it disabled it?
>> >>
>> >> More generally, can't we for example blow away the
>> >> indir_desc array that we use to keep the ctx pointers?
>> >> Won't that be enough?
>> >
>> > I'm not sure how it is related to the indirect descriptor but an
>> > example is that all the current driver will assume:
>> >
>> > 1) the interrupt won't be raised before virtio_device_ready()
>> > 2) the interrupt won't be raised after reset()
>> 
>> If that assumption exists, then you better keep the interrupt line
>> disabled until virtio_device_ready() has completed
>
> started not completed. device is allowed to send
> config interrupts right after DRIVER_OK status is set by
> virtio_device_ready.

Whatever:

 * Define the exact point from which on the driver is able to handle the
   interrupt and put the enable after that point

 * Define the exact point from which on the driver is unable to handle
   the interrupt and put the disable before that point

The above is blury.

>> and disable it again
>> before reset() is invoked. That's a question of general robustness and
>> not really a question of trusted hypervisors and encrypted guests.
>
> We can do this for some MSIX interrupts, sure. Not for shared interrupts though.

See my reply to the next patch. The problem is the same:

 * Define the exact point from which on the driver is able to handle the
   interrupt and allow the handler to proceed after that point

 * Define the exact point from which on the driver is unable to handle
   the interrupt and ensure that the handler denies to proceed before
   that point

Same story just a different mechanism.

Thanks,

        tglx