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Message-ID: <cf755bb8-4265-875f-dc20-eefc0e8740f4@suse.com>
Date: Thu, 16 Jun 2022 10:56:18 +0200
From: Juergen Gross <jgross@...e.com>
To: Oleksandr <olekstysh@...il.com>
Cc: xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
viresh.kumar@...aro.org, hch@...radead.org,
Stefano Stabellini <sstabellini@...nel.org>,
Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
Subject: Re: [PATCH v2] xen: don't require virtio with grants for non-PV
guests
On 16.06.22 09:31, Oleksandr wrote:
>
> On 16.06.22 08:37, Juergen Gross wrote:
>
>
> Hello Juergen
>
>> Commit fa1f57421e0b ("xen/virtio: Enable restricted memory access using
>> Xen grant mappings") introduced a new requirement for using virtio
>> devices: the backend now needs to support the VIRTIO_F_ACCESS_PLATFORM
>> feature.
>>
>> This is an undue requirement for non-PV guests, as those can be operated
>> with existing backends without any problem, as long as those backends
>> are running in dom0.
>>
>> Per default allow virtio devices without grant support for non-PV
>> guests.
>>
>> Add a new config item to always force use of grants for virtio.
>>
>> Fixes: fa1f57421e0b ("xen/virtio: Enable restricted memory access using Xen
>> grant mappings")
>> Reported-by: Viresh Kumar <viresh.kumar@...aro.org>
>> Signed-off-by: Juergen Gross <jgross@...e.com>
>> ---
>> V2:
>> - remove command line parameter (Christoph Hellwig)
>> ---
>> drivers/xen/Kconfig | 9 +++++++++
>> include/xen/xen.h | 2 +-
>> 2 files changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
>> index bfd5f4f706bc..a65bd92121a5 100644
>> --- a/drivers/xen/Kconfig
>> +++ b/drivers/xen/Kconfig
>> @@ -355,4 +355,13 @@ config XEN_VIRTIO
>> If in doubt, say n.
>> +config XEN_VIRTIO_FORCE_GRANT
>> + bool "Require Xen virtio support to use grants"
>> + depends on XEN_VIRTIO
>> + help
>> + Require virtio for Xen guests to use grant mappings.
>> + This will avoid the need to give the backend the right to map all
>> + of the guest memory. This will need support on the backend side
>> + (e.g. qemu or kernel, depending on the virtio device types used).
>> +
>> endmenu
>> diff --git a/include/xen/xen.h b/include/xen/xen.h
>> index 0780a81e140d..4d4188f20337 100644
>> --- a/include/xen/xen.h
>> +++ b/include/xen/xen.h
>> @@ -56,7 +56,7 @@ extern u64 xen_saved_max_mem_size;
>> static inline void xen_set_restricted_virtio_memory_access(void)
>> {
>> - if (IS_ENABLED(CONFIG_XEN_VIRTIO) && xen_domain())
>> + if (IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT) || xen_pv_domain())
>> platform_set(PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS);
>
>
> Looks like, the flag will be *always* set for paravirtualized guests even if
> CONFIG_XEN_VIRTIO disabled.
>
> Maybe we should clarify the check?
>
>
> if (IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT) || IS_ENABLED(CONFIG_XEN_VIRTIO)
> && xen_pv_domain())
>
> platform_set(PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS);
>
Yes, we should. I had the function in grant-dma-ops.c in V1, and could drop the
CONFIG_XEN_VIRTIO dependency for that reason.
I'll wait for more comments before sending V3, though.
Juergen
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