Re: [PATCH v2 05/12] x86/hyperv: Change vTOM handling to use standard coco mechanisms

Tom Lendacky
2022-11-16T16:43:00Z

On 11/13/22 10:01, Michael Kelley (LINUX) wrote:
> From: Tom Lendacky <thomas.lendacky@amd.com> Sent: Friday, November 11, 2022 10:50 AM
>>
>> On 11/11/22 00:21, Michael Kelley wrote:
>
> [snip]
>
>>> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
>>> index 06eb8910..024fbf4 100644
>>> --- a/arch/x86/mm/pat/set_memory.c
>>> +++ b/arch/x86/mm/pat/set_memory.c
>>> @@ -2126,10 +2126,8 @@ static int __set_memory_enc_pgtable(unsigned long
>> addr, int numpages, bool enc)
>>>
>>> static int __set_memory_enc_dec(unsigned long addr, int numpages, bool enc)
>>> {
>>> - if (hv_is_isolation_supported())
>>> - return hv_set_mem_host_visibility(addr, numpages, !enc);
>>> -
>>> - if (cc_platform_has(CC_ATTR_MEM_ENCRYPT))
>>> + if (cc_platform_has(CC_ATTR_MEM_ENCRYPT) ||
>>> + cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT))
>>
>> This seems kind of strange since CC_ATTR_MEM_ENCRYPT is supposed to mean
>> either HOST or GUEST memory encryption, but then you check for GUEST
>> memory encryption directly. Can your cc_platform_has() support be setup to
>> handle the CC_ATTR_MEM_ENCRYPT attribute in some way?
>>
>> Thanks,
>> Tom
>
> Current upstream code for Hyper-V guests with vTOM enables only
> CC_ATTR_GUEST_MEM_ENCRYPT. I had been wary of also enabling
> CC_ATTR_MEM_ENCRYPT because that would enable other code paths that
> Might not be right for the vTOM case. But looking at it more closely, enabling
> CC_ATTR_MEM_ENCRYPT may work.
>
> There are two problems with Hyper-V vTOM enabling CC_ATTR_MEM_ENCRYPT,
> but both are fixable:
>
> 1) The call to mem_encrypt_init() happens a little bit too soon. Hyper-V is fully
> initialized and hypercalls become possible after start_kernel() calls late_time_init().
> mem_encrypt_init() needs to happen after the call to late_time_init() so that
> marking the swiotlb memory as decrypted can make the hypercalls to sync the
> page state change with the host. Moving mem_encrypt_init() a few lines later in
> start_kernel() works in my case, but I can't test all the cases that you probably
> have. This change also has the benefit of removing the call to
> swiotlb_update_mem_attributes() at the end of hyperv_init(), which always
> seemed like a hack.

It seems safe for SME/SEV since mem_encrypt_init() is only updating the
SWIOTLB attributes at this point. I'll do some quick testing, but you
might want to verify with TDX folks, too.

>
> 2) mem_encrypt_free_decrypted_mem() is mismatched with
> sme_postprocess_startup() in its handling of bss decrypted memory. The
> decryption is done if sme_me_mask is non-zero, while the re-encryption is
> done if CC_ATTR_MEM_ENCRYPT is true, and those conditions won't be
> equivalent in a Hyper-V vTOM VM if we enable CC_ATTR_MEM_ENCRYPT
> (sme_me_mask is always zero in a Hyper-V vTOM VM). Changing
> mem_encrypt_free_decrypted_mem() to do re-encryption only if sme_me_mask
> is non-zero solves that problem. Note that there doesn't seem to be a way for a

Hmmm, yes, this was because of an issue using the cc_platform_has() call
during identity mapped paging. I think matching them in this case would be
best, e.g., changing mem_encrypt_free_decrypted_mem() to check for a
non-zero sme_me_mask - along with a nice comment on why it is checking
sme_me_mask.

Thanks,
Tom

> Hyper-V vTOM VM to have decrypted bss, since there's no way to sync the
> page state change with the host that early in the boot process, but I don't think
> there's a requirement for such, so all is good.
>
> With the above two changes, Hyper-V vTOM VMs can enable
> CC_ATTR_MEM_ENCRYPT. The Hyper-V hack in __set_memory_enc_dec()
> still goes away, and there's no change to the condition for invoking
> __set_memory_enc_pgtable().
>
> Thoughts? Have I missed anything? Overall, I'm persuaded that this is a better
> approach and can submit a v3 patch series with these changes if you agree.
>
> Michael