Re: [Patch v3 07/14] x86/hyperv: Change vTOM handling to use standard coco mechanisms

[Tianyu Lan]

On 11/17/2022 2:41 AM, 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()
>
> * Remove the Hyper-V specific call to swiotlb_update_mem_attributes()
>    since mem_encrypt_init() will now do it.
>
> [1]https://lore.kernel.org/all/20211025122116.264793-1-ltykernel@xxxxxxxxx/
> [2]https://lore.kernel.org/all/20211213071407.314309-1-ltykernel@xxxxxxxxx/
>
> Signed-off-by: Michael Kelley<mikelley@xxxxxxxxxxxxx>
> ---
>   arch/x86/coco/core.c            | 11 +++++++++-
>   arch/x86/hyperv/hv_init.c       | 11 ----------
>   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    |  3 ---
>   6 files changed, 53 insertions(+), 40 deletions(-)

Reviewed-by: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>