On 10/21/2022 1:57 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(), 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@xxxxxxxxx/ [2]https://lore.kernel.org/all/20211213071407.314309-1-ltykernel@xxxxxxxxx/ Signed-off-by: Michael Kelley<mikelley@xxxxxxxxxxxxx> ---
Reviewed-by: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>
