Re: [PATCH 2/2] virtio: replace arch_has_restricted_virtio_memory_access()

[Tom Lendacky <thomas.lendacky@amd.com>]

On 4/27/22 07:37, Juergen Gross wrote:
> On 27.04.22 14:28, Borislav Petkov wrote:
>> On Wed, Apr 27, 2022 at 08:37:31AM +0200, Juergen Gross wrote:
>>> On 26.04.22 19:35, Borislav Petkov wrote:
>>>> On Tue, Apr 26, 2022 at 03:40:21PM +0200, Juergen Gross wrote:
>>>>>    /* protected virtualization */
>>>>>    static void pv_init(void)
>>>>>    {
>>>>>        if (!is_prot_virt_guest())
>>>>>            return;
>>>>> +    platform_set_feature(PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS);
>>>>
>>>> Kinda long-ish for my taste. I'll probably call it:
>>>>
>>>>     platform_set()
>>>>
>>>> as it is implicit that it sets a feature bit.
>>>
>>> Okay, fine with me.
>>>
>>>>
>>>>> diff --git a/arch/x86/mm/mem_encrypt_identity.c 
>>>>> b/arch/x86/mm/mem_encrypt_identity.c
>>>>> index b43bc24d2bb6..6043ba6cd17d 100644
>>>>> --- a/arch/x86/mm/mem_encrypt_identity.c
>>>>> +++ b/arch/x86/mm/mem_encrypt_identity.c
>>>>> @@ -40,6 +40,7 @@
>>>>>    #include <linux/mm.h>
>>>>>    #include <linux/mem_encrypt.h>
>>>>>    #include <linux/cc_platform.h>
>>>>> +#include <linux/platform-feature.h>
>>>>>    #include <asm/setup.h>
>>>>>    #include <asm/sections.h>
>>>>> @@ -566,6 +567,10 @@ void __init sme_enable(struct boot_params *bp)
>>>>>        } else {
>>>>>            /* SEV state cannot be controlled by a command line option */
>>>>>            sme_me_mask = me_mask;
>>>>> +
>>>>> +        /* Set restricted memory access for virtio. */
>>>>> +        platform_set_feature(PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS);

This is way early in the boot, but it appears that marking the platform 
feature bitmap as __read_mostly puts this in the .data section, so avoids 
the issue of bss being cleared.

TDX support also uses the arch_has_restricted_virtio_memory_access() 
function and will need to be updated.

Seems like a lot of changes, I just wonder if the the arch_has...() 
function couldn't be updated to also include a Xen check?

Thanks,
Tom

>>>>
>>>> Huh, what does that have to do with SME?
>>>
>>> I picked the function where sev_status is being set, as this seemed to be
>>> the correct place to set the feature bit.
>>
>> What I don't understand is what does restricted memory access have to do
>> with AMD SEV and how does play together with what you guys are trying to
>> do?
>>
>> The big picture pls.
> 
> Ah, okay.
> 
> For support of virtio with Xen we want to not only support the virtio
> devices like KVM, but use grants for letting the guest decide which
> pages are allowed to be mapped by the backend (dom0).
> 
> Instead of physical guest addresses the guest will use grant-ids (plus
> offset). In order to be able to handle this at the basic virtio level
> instead of the single virtio device drivers, we need to use dedicated
> dma-ops. And those will be used by virtio only, if the "restricted
> virtio memory request" flag is set, which is used by SEV, too. In order
> to let virtio set this flag, we need a way to communicate to virtio
> that the running system is either a SEV guest or a Xen guest.
> 
> HTH,
> 
> 
> Juergen