Messages in this thread | | | Date | Wed, 9 Nov 2022 09:09:40 +0800 | Subject | Re: [PATCH 05/12] x86/hyperv: Change vTOM handling to use standard coco mechanisms | From | Tianyu Lan <> |
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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@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>
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