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Subject: Re: [PATCH v7 2/2] s390: virtio: PV needs VIRTIO I/O device
 protection
To:     "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>
Cc:     linux-kernel@vger.kernel.org, pasic@linux.ibm.com,
        borntraeger@de.ibm.com, frankja@linux.ibm.com, cohuck@redhat.com,
        kvm@vger.kernel.org, linux-s390@vger.kernel.org,
        virtualization@lists.linux-foundation.org, thomas.lendacky@amd.com,
        david@gibson.dropbear.id.au, linuxram@us.ibm.com,
        hca@linux.ibm.com, gor@linux.ibm.com
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From:   Pierre Morel <pmorel@linux.ibm.com>
Message-ID: <be992b00-de1d-4499-ee7f-b2b2b5a8879d@linux.ibm.com>
Date:   Wed, 22 Jul 2020 13:48:24 +0200
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On 2020-07-15 13:51, Michael S. Tsirkin wrote:
> On Wed, Jul 15, 2020 at 06:16:59PM +0800, Jason Wang wrote:
>>
>> On 2020/7/15 下午5:50, Michael S. Tsirkin wrote:
>>> On Wed, Jul 15, 2020 at 10:31:09AM +0200, Pierre Morel wrote:
>>>> If protected virtualization is active on s390, the virtio queues are
>>>> not accessible to the host, unless VIRTIO_F_IOMMU_PLATFORM has been
>>>> negotiated. Use the new arch_validate_virtio_features() interface to
>>>> fail probe if that's not the case, preventing a host error on access
>>>> attempt.
>>>>
>>>> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
>>>> Reviewed-by: Cornelia Huck <cohuck@redhat.com>
>>>> Acked-by: Halil Pasic <pasic@linux.ibm.com>
>>>> Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
>>>> ---
>>>>    arch/s390/mm/init.c | 28 ++++++++++++++++++++++++++++
>>>>    1 file changed, 28 insertions(+)
>>>>
>>>> diff --git a/arch/s390/mm/init.c b/arch/s390/mm/init.c
>>>> index 6dc7c3b60ef6..d39af6554d4f 100644
>>>> --- a/arch/s390/mm/init.c
>>>> +++ b/arch/s390/mm/init.c
>>>> @@ -45,6 +45,7 @@
>>>>    #include <asm/kasan.h>
>>>>    #include <asm/dma-mapping.h>
>>>>    #include <asm/uv.h>
>>>> +#include <linux/virtio_config.h>
>>>>    pgd_t swapper_pg_dir[PTRS_PER_PGD] __section(.bss..swapper_pg_dir);
>>>> @@ -161,6 +162,33 @@ bool force_dma_unencrypted(struct device *dev)
>>>>    	return is_prot_virt_guest();
>>>>    }
>>>> +/*
>>>> + * arch_validate_virtio_features
>>>> + * @dev: the VIRTIO device being added
>>>> + *
>>>> + * Return an error if required features are missing on a guest running
>>>> + * with protected virtualization.
>>>> + */
>>>> +int arch_validate_virtio_features(struct virtio_device *dev)
>>>> +{
>>>> +	if (!is_prot_virt_guest())
>>>> +		return 0;
>>>> +
>>>> +	if (!virtio_has_feature(dev, VIRTIO_F_VERSION_1)) {
>>>> +		dev_warn(&dev->dev,
>>>> +			 "legacy virtio not supported with protected virtualization\n");
>>>> +		return -ENODEV;
>>>> +	}
>>>> +
>>>> +	if (!virtio_has_feature(dev, VIRTIO_F_IOMMU_PLATFORM)) {
>>>> +		dev_warn(&dev->dev,
>>>> +			 "support for limited memory access required for protected virtualization\n");
>>>> +		return -ENODEV;
>>>> +	}
>>>> +
>>>> +	return 0;
>>>> +}
>>>> +
>>>>    /* protected virtualization */
>>>>    static void pv_init(void)
>>>>    {
>>> What bothers me here is that arch code depends on virtio now.
>>> It works even with a modular virtio when functions are inline,
>>> but it seems fragile: e.g. it breaks virtio as an out of tree module,
>>> since layout of struct virtio_device can change.
>>
>>
>> The code was only called from virtio.c so it should be fine.
>>
>> And my understanding is that we don't need to care about the kABI issue
>> during upstream development?
>>
>> Thanks
> 
> No, but so far it has been convenient at least for me, for development,
> to just be able to unload all of virtio and load a different version.
> 
> 
>>
>>>
>>> I'm not sure what to do with this yet, will try to think about it
>>> over the weekend. Thanks!
>>>
>>>
>>>> -- 
>>>> 2.25.1
> 

Hi Michael,

I am not sure to understand the problem so I may propose a wrong 
solution but, let's try:

Would a callback registration instead of a weak function solve the problem?
The registrating function in core could test a parameter to check if the 
callback is in sync with the VIRTIO core.

What do you think?

Regards,
Pierre


-- 
Pierre Morel
IBM Lab Boeblingen