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Re: [Xen-devel] [PATCH v3] x86/HVM: p2m_ram_ro is incompatible with device pass-through


  • To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Tue, 1 Oct 2019 19:00:20 +0100
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  • Cc: Paul Durrant <paul@xxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Tue, 01 Oct 2019 18:00:30 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 01/10/2019 10:07, Jan Beulich wrote:
> The write-discard property of the type can't be represented in IOMMU
> page table entries. Make sure the respective checks / tracking can't
> race, by utilizing the domain lock. The other sides of the sharing/
> paging/log-dirty exclusion checks should subsequently perhaps also be
> put under that lock then.
>
> This also fixes an unguarded d->arch.hvm access.
>
> Take the opportunity and also convert neighboring bool_t to bool in
> struct hvm_domain.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Reviewed-by: Paul Durrant <paul.durrant@xxxxxxxxxx>

Leaving aside the logdirty change which is an automatic no from me in
this form,  I can't see how this change avoids regressing the status quo.

Qemu really does set this flag for option ROMs, which will typically be
the iPXE image for net booting.  Currently, netbooting an HVM guest with
device passed through to it does work, because it is part of XenServers
basic testing.

It is entirely possible that this only "works" as long as DMA doesn't
touch the option ROM, and while this might not be ideal, it seems to be
less bad behaviour than breaking existing VMs using this configuration.

~Andrew

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