linux-kernel - Re: [PATCH v2] xen: don't require virtio with grants for non-PV guests

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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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