Re: [Patch v4 06/13] x86/hyperv: Change vTOM handling to use standard coco mechanisms

Borislav Petkov
2023-01-09T16:38:46Z

On Thu, Dec 01, 2022 at 07:30:24PM -0800, 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 0x400000000000 (bit 46 is set). Guest physical
> 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 IO-APIC to use private accesses.
>
> To accomplish the switch in approach, the following must be done in
> this single patch:

s/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
>
> * Remove CC_VENDOR_HYPERV and merge the associated vTOM functionality
> under CC_VENDOR_AMD. Update cc_mkenc() and cc_mkdec() to set/clear
> the vTOM bit as a protection flag.
>
> * 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@gmail.com/
> [2] https://lore.kernel.org/all/20211213071407.314309-1-ltykernel@gmail.com/
>
> Signed-off-by: Michael Kelley <mikelley@microsoft.com>
> ---
> arch/x86/coco/core.c | 37 ++++++++++++++++++++--------
> arch/x86/hyperv/hv_init.c | 11 ---------
> arch/x86/hyperv/ivm.c | 52 +++++++++++++++++++++++++++++++---------
> arch/x86/include/asm/coco.h | 1 -
> arch/x86/include/asm/mshyperv.h | 8 ++-----
> arch/x86/include/asm/msr-index.h | 1 +
> arch/x86/kernel/cpu/mshyperv.c | 15 ++++++------
> arch/x86/mm/pat/set_memory.c | 3 ---
> 8 files changed, 78 insertions(+), 50 deletions(-)
>
> diff --git a/arch/x86/coco/core.c b/arch/x86/coco/core.c
> index 49b44f8..c361c52 100644
> --- a/arch/x86/coco/core.c
> +++ b/arch/x86/coco/core.c
> @@ -44,6 +44,24 @@ static bool intel_cc_platform_has(enum cc_attr attr)
> static bool amd_cc_platform_has(enum cc_attr attr)
> {
> #ifdef CONFIG_AMD_MEM_ENCRYPT
> +
> + /*
> + * Handle the SEV-SNP vTOM case where sme_me_mask must be zero,
> + * and the other levels of SME/SEV functionality, including C-bit
> + * based SEV-SNP, must not be enabled.
> + */
> + if (sev_status & MSR_AMD64_SNP_VTOM_ENABLED) {

return amd_cc_platform_vtom();

or so and then stick that switch in there.

This way it looks kinda grafted in front and with a function call with a telling
name it says it is a special case...

> + switch (attr) {
> + case CC_ATTR_GUEST_MEM_ENCRYPT:
> + case CC_ATTR_MEM_ENCRYPT:
> + case CC_ATTR_ACCESS_IOAPIC_ENCRYPTED:
> + return true;
> + default:
> + return false;
> + }
> + }

The rest looks kinda nice, I gotta say. I can't complain. :)

Thx.

--
Regards/Gruss,
Boris.