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

Michael Kelley (LINUX)
2022-11-13T16:01:54Z

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.

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
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