x86/sev/docs: Document the SNP Reverse Map Table (RMP)

Update the AMD memory encryption documentation to include information on
the Reverse Map Table (RMP) and the two table formats.

Signed-off-by: Tom Lendacky <[e-mail]>
Signed-off-by: Borislav Petkov (AMD) <[e-mail]>
Reviewed-by: Nikunj A Dadhania <[e-mail]>
Reviewed-by: Neeraj Upadhyay <[e-mail]>
Link: https://lore.kernel.org/r/d3feea5[e-mail]

Documentation/arch/x86/amd-memory-encryption.rst

···
 
 More details in AMD64 APM[1] Vol 2: 15.34.10 SEV_STATUS MSR
 
+Reverse Map Table (RMP)
+=======================
+
+The RMP is a structure in system memory that is used to ensure a one-to-one
+mapping between system physical addresses and guest physical addresses. Each
+page of memory that is potentially assignable to guests has one entry within
+the RMP.
+
+The RMP table can be either contiguous in memory or a collection of segments
+in memory.
+
+Contiguous RMP
+--------------
+
+Support for this form of the RMP is present when support for SEV-SNP is
+present, which can be determined using the CPUID instruction::
+
+	0x8000001f[eax]:
+		Bit[4] indicates support for SEV-SNP
+
+The location of the RMP is identified to the hardware through two MSRs::
+
+        0xc0010132 (RMP_BASE):
+                System physical address of the first byte of the RMP
+
+        0xc0010133 (RMP_END):
+                System physical address of the last byte of the RMP
+
+Hardware requires that RMP_BASE and (RPM_END + 1) be 8KB aligned, but SEV
+firmware increases the alignment requirement to require a 1MB alignment.
+
+The RMP consists of a 16KB region used for processor bookkeeping followed
+by the RMP entries, which are 16 bytes in size. The size of the RMP
+determines the range of physical memory that the hypervisor can assign to
+SEV-SNP guests. The RMP covers the system physical address from::
+
+        0 to ((RMP_END + 1 - RMP_BASE - 16KB) / 16B) x 4KB.
+
+The current Linux support relies on BIOS to allocate/reserve the memory for
+the RMP and to set RMP_BASE and RMP_END appropriately. Linux uses the MSR
+values to locate the RMP and determine the size of the RMP. The RMP must
+cover all of system memory in order for Linux to enable SEV-SNP.
+
+Segmented RMP
+-------------
+
+Segmented RMP support is a new way of representing the layout of an RMP.
+Initial RMP support required the RMP table to be contiguous in memory.
+RMP accesses from a NUMA node on which the RMP doesn't reside
+can take longer than accesses from a NUMA node on which the RMP resides.
+Segmented RMP support allows the RMP entries to be located on the same
+node as the memory the RMP is covering, potentially reducing latency
+associated with accessing an RMP entry associated with the memory. Each
+RMP segment covers a specific range of system physical addresses.
+
+Support for this form of the RMP can be determined using the CPUID
+instruction::
+
+        0x8000001f[eax]:
+                Bit[23] indicates support for segmented RMP
+
+If supported, segmented RMP attributes can be found using the CPUID
+instruction::
+
+        0x80000025[eax]:
+                Bits[5:0]  minimum supported RMP segment size
+                Bits[11:6] maximum supported RMP segment size
+
+        0x80000025[ebx]:
+                Bits[9:0]  number of cacheable RMP segment definitions
+                Bit[10]    indicates if the number of cacheable RMP segments
+                           is a hard limit
+
+To enable a segmented RMP, a new MSR is available::
+
+        0xc0010136 (RMP_CFG):
+                Bit[0]     indicates if segmented RMP is enabled
+                Bits[13:8] contains the size of memory covered by an RMP
+                           segment (expressed as a power of 2)
+
+The RMP segment size defined in the RMP_CFG MSR applies to all segments
+of the RMP. Therefore each RMP segment covers a specific range of system
+physical addresses. For example, if the RMP_CFG MSR value is 0x2401, then
+the RMP segment coverage value is 0x24 => 36, meaning the size of memory
+covered by an RMP segment is 64GB (1 << 36). So the first RMP segment
+covers physical addresses from 0 to 0xF_FFFF_FFFF, the second RMP segment
+covers physical addresses from 0x10_0000_0000 to 0x1F_FFFF_FFFF, etc.
+
+When a segmented RMP is enabled, RMP_BASE points to the RMP bookkeeping
+area as it does today (16K in size). However, instead of RMP entries
+beginning immediately after the bookkeeping area, there is a 4K RMP
+segment table (RST). Each entry in the RST is 8-bytes in size and represents
+an RMP segment::
+
+        Bits[19:0]  mapped size (in GB)
+                    The mapped size can be less than the defined segment size.
+                    A value of zero, indicates that no RMP exists for the range
+                    of system physical addresses associated with this segment.
+        Bits[51:20] segment physical address
+                    This address is left shift 20-bits (or just masked when
+                    read) to form the physical address of the segment (1MB
+                    alignment).
+
+The RST can hold 512 segment entries but can be limited in size to the number
+of cacheable RMP segments (CPUID 0x80000025_EBX[9:0]) if the number of cacheable
+RMP segments is a hard limit (CPUID 0x80000025_EBX[10]).
+
+The current Linux support relies on BIOS to allocate/reserve the memory for
+the segmented RMP (the bookkeeping area, RST, and all segments), build the RST
+and to set RMP_BASE, RMP_END, and RMP_CFG appropriately. Linux uses the MSR
+values to locate the RMP and determine the size and location of the RMP
+segments. The RMP must cover all of system memory in order for Linux to enable
+SEV-SNP.
+
+More details in the AMD64 APM Vol 2, section "15.36.3 Reverse Map Table",
+docID: 24593.
+
 Secure VM Service Module (SVSM)
 ===============================
+
 SNP provides a feature called Virtual Machine Privilege Levels (VMPL) which
 defines four privilege levels at which guest software can run. The most
 privileged level is 0 and numerically higher numbers have lesser privileges.