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

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

On 11/11/22 00:21, Michael Kelley wrote:
> Hyper-V guests on AMD SEV-SNP hardware have the option of using the
> "virtual Top Of Memory" (vTOM) feature specified by the SEV-SNP
> architecture. With vTOM, shared vs. private memory accesses are
> controlled by splitting the guest physical address space into two
> halves.  vTOM is the dividing line where the uppermost bit of the
> physical address space is set; e.g., with 47 bits of guest physical
> address space, vTOM is 0x40000000000 (bit 46 is set).  Guest phyiscal
> memory is accessible at two parallel physical addresses -- one below
> vTOM and one above vTOM.  Accesses below vTOM are private (encrypted)
> while accesses above vTOM are shared (decrypted). In this sense, vTOM
> is like the GPA.SHARED bit in Intel TDX.
> 
> Support for Hyper-V guests using vTOM was added to the Linux kernel in
> two patch sets[1][2]. This support treats the vTOM bit as part of
> the physical address. For accessing shared (decrypted) memory, these
> patch sets create a second kernel virtual mapping that maps to physical
> addresses above vTOM.
> 
> A better approach is to treat the vTOM bit as a protection flag, not
> as part of the physical address. This new approach is like the approach
> for the GPA.SHARED bit in Intel TDX. Rather than creating a second kernel
> virtual mapping, the existing mapping is updated using recently added
> coco mechanisms.  When memory is changed between private and shared using
> set_memory_decrypted() and set_memory_encrypted(), the PTEs for the
> existing kernel mapping are changed to add or remove the vTOM bit
> in the guest physical address, just as with TDX. The hypercalls to
> change the memory status on the host side are made using the existing
> callback mechanism. Everything just works, with a minor tweak to map
> the I/O APIC to use private accesses.
> 
> To accomplish the switch in approach, the following must be done in
> in this single patch:
> 
> * Update Hyper-V initialization to set the cc _mask based on vTOM
>    and do other coco initialization.
> 
> * Update physical_mask so the vTOM bit is no longer treated as part
>    of the physical address
> 
> * Update cc_mkenc() and cc_mkdec() to be active for Hyper-V guests.
>    This makes the vTOM bit part of the protection flags.
> 
> * Code already exists to make hypercalls to inform Hyper-V about pages
>    changing between shared and private.  Update this code to run as a
>    callback from __set_memory_enc_pgtable().
> 
> * Remove the Hyper-V special case from __set_memory_enc_dec(), and
>    make the normal case active for Hyper-V VMs, which have
>    CC_ATTR_GUEST_MEM_ENCRYPT, but not CC_ATTR_MEM_ENCRYPT.
> 
> [1] https://lore.kernel.org/all/20211025122116.264793-1-ltykernel@gmail.com/
> [2] https://lore.kernel.org/all/20211213071407.314309-1-ltykernel@gmail.com/
> 
> Signed-off-by: Michael Kelley <mikelley@microsoft.com>
> Reviewed-by: Tianyu Lan <Tianyu.Lan@microsoft.com>
> ---
>   arch/x86/coco/core.c            | 10 ++++++++-
>   arch/x86/hyperv/ivm.c           | 45 +++++++++++++++++++++++++++++++----------
>   arch/x86/include/asm/mshyperv.h |  8 ++------
>   arch/x86/kernel/cpu/mshyperv.c  | 15 +++++++-------
>   arch/x86/mm/pat/set_memory.c    |  6 ++----
>   5 files changed, 54 insertions(+), 30 deletions(-)
> 


> 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

>   		return __set_memory_enc_pgtable(addr, numpages, enc);
>   
>   	return 0;