From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marcelo Tosatti <mtosatti@redhat.com>
Subject: Re: qemu/hw/device-assignment: questions about msix_table_page
Date: Tue, 5 May 2009 07:19:45 -0300
Message-ID: <20090505101945.GA11426@amt.cnet>
References: <20090427104117.GB29082@redhat.com> <200904272203.59909.sheng@linux.intel.com> <20090427141504.GC2504@redhat.com> <200904272230.18253.sheng@linux.intel.com> <20090505095136.GA12797@redhat.com>
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Cc: Sheng Yang <sheng@linux.intel.com>, Avi Kivity <avi@redhat.com>,
	kvm@vger.kernel.org
To: "Michael S. Tsirkin" <mst@redhat.com>
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On Tue, May 05, 2009 at 12:51:36PM +0300, Michael S. Tsirkin wrote:
> On Mon, Apr 27, 2009 at 10:30:17PM +0800, Sheng Yang wrote:
> > > > > > If guest can write to the real device MSI-X table directly, it would
> > > > > > cause chaos on interrupt delivery, for what guest see is totally
> > > > > > different with what's host see...
> > > > >
> > > > > Obviously.
> > > > >
> > > > > Thanks,
> > 
> 
> What's the reason that this page is unmapped from the qemu memory space?
> Specifically what do these lines do:
>             int offset = r_dev->msix_table_addr - real_region->base_addr;
>             ret = munmap(region->u.r_virtbase + offset, TARGET_PAGE_SIZE);

I believe this allows accesses to this page (the MSI-X table), which
is part of the guest address space (through kvm memory slots), to be
trapped by qemu.

Since there is no actual page in this guest address, KVM treats accesses
as MMIO and forwards them to QEMU.