kvm device assignment and MSI-X masking

From: Jan Kiszka <jan.kiszka@siemens.com>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: kvm <kvm@vger.kernel.org>
Subject: kvm device assignment and MSI-X masking
Date: Tue, 14 Aug 2012 15:48:26 +0200	[thread overview]
Message-ID: <502A572A.5040408@siemens.com> (raw)

Hi Alex,

you once wrote this comment in device-assignment.c, msix_mmio_write:

    if (!msix_masked(&orig) && msix_masked(entry)) {
        /*
         * Vector masked, disable it
         *
         * XXX It's not clear if we can or should actually attempt
         * to mask or disable the interrupt.  KVM doesn't have
         * support for pending bits and kvm_assign_set_msix_entry
         * doesn't modify the device hardware mask.  Interrupts
         * while masked are simply not injected to the guest, so
         * are lost.  Can we get away with always injecting an
         * interrupt on unmask?
         */

I'm wondering what made you think that we won't inject if the vector is
masked like this (ie. in the shadow MSI-X table). Can you recall the
details?

I'm trying to refactor this code to make the KVM interface a bit more
encapsulating the kernel interface details, not fixing anything. Still,
I would also like to avoid introducing regressions.

Jan

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