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Re: PVH dom0 and MSIX memory mapping issue
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Fri, 25 Feb 2022 16:46:56 +0100
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- Cc: Alex Olson <this.is.a0lson@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Fri, 25 Feb 2022 15:47:20 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Fri, Feb 25, 2022 at 03:22:59PM +0100, Jan Beulich wrote:
> On 25.02.2022 14:50, Alex Olson wrote:
> > I realize PVH for dom0 is still experimental, but was trying to see how
> > well it
> > works in the state of "master".
> >
> > I found one issue with MSI-X interrupts in dom0 -- a fatal page fault occurs
> > when the MSI-X PBA is accessed from dom0. It looks like dom0 doesn't have
> > an
> > identity mapping for the PBA of a PCI device -- intentionally caused by
> > vpci_make_msix_hole() ?
> >
> > I was also wondering, what is the impact of "existing mapping (...) at 0
> > clobbers MSIX MMIO area".
> >
> > Is the purpose of vpci_make_msix_hole() to make MSIX table/PBA accesses
> > trap to
> > the hypervisor? Seems like the page-based granularity is too coarse since
> > the
> > PBA can be much smaller than a page as well as not aligned...
Page-size granularity is the minimum that we can do in the second
stage page tables.
I guess we could do with some logic to forward accesses outside of the
MSI-X table and PBA that falls into a valid BAR region that shared the
4K page.
> > I was able to get something potentially usable by adding a "ioremap" call in
> > msix_read() and msix_write to deal with the access of the PBA. Wasn't
> > sure if
> > this was a proper way of handling things... Was also wondering if it would
> > make
> > sense to do the same for the "clobbers MSIX MMIO area" cases too.
>
> You happen to ask just over two hours after a patch was posted:
> https://lists.xen.org/archives/html/xen-devel/2022-02/msg01657.html
> Since Roger didn't have an environment to test his change, maybe you
> could do so, offering a Tested-by if successful?
I had to post a new verison because I idnd't recall that for dom0 we
also allow writes to the PBA, it's here:
https://lore.kernel.org/xen-devel/20220225153956.1078-1-roger.pau@xxxxxxxxxx/
Roger.
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