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docs: PROTOCOL.md: Backport caching section from trunk

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mintsuki 2023-09-16 16:31:16 -05:00
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@ -112,6 +112,57 @@ If the kernel is a position independent executable, the bootloader is free to
relocate it as it sees fit, potentially performing KASLR (as specified by the
config).
## Caching
### x86-64
The kernel executable, loaded at or above `0xffffffff80000000`, sees all of its
segments mapped using write-back (WB) caching at the page tables level.
All HHDM and identity map memory regions are mapped using write-back (WB) caching at the page
tables level, except framebuffer regions which are mapped using write-combining
(WC) caching at the page tables level.
The PAT's (Page Attribute Table) layout is specified to be as follows:
```
PAT0 -> WB
PAT1 -> WT
PAT2 -> UC-
PAT3 -> UC
PAT4 -> WP
PAT5 -> WC
PAT6 -> unspecified
PAT7 -> unspecified
```
The MTRRs are left as the firmware set them up.
### aarch64
The kernel executable, loaded at or above `0xffffffff80000000`, sees all of its
segments mapped using Normal Write-Back RW-Allocate non-transient caching mode.
All HHDM and identity map memory regions are mapped using the Normal Write-Back RW-Allocate
non-transient caching mode, except for the framebuffer regions, which are
mapped in using an unspecified caching mode, correct for use with the
framebuffer on the platform.
The `MAIR_EL1` register will at least contain entries for the above-mentioned
caching modes, in an unspecified order.
In order to access MMIO regions, the kernel must ensure the correct caching mode
is used on its own.
### riscv64
If the `Svpbmt` extension is available, all framebuffer memory regions are mapped
with `PBMT=NC` to enable write-combining optimizations. The kernel executable,
loaded at or above `0xffffffff80000000`, and all HHDM and identity map memory regions are mapped
with the default `PBMT=PMA`.
If the `Svpbmt` extension is not available, no PMAs can be overridden (effectively,
everything is mapped with `PBMT=PMA`).
## Machine state at entry
### x86-64
@ -163,27 +214,25 @@ All other general purpose registers are set to 0.
unless the an Entry Point feature is requested (see below), in which case,
the value of `PC` is going to be taken from there.
The contents of the `VBAR_EL1` register are undefined, and the kernel must load it's own.
The contents of the `VBAR_EL1` register are undefined, and the kernel must load
its own.
The `MAIR_EL1` register will contain at least these entries, in an unspecified order:
- Normal, Write-back RW-Allocate non-transient (`0b11111111`),
- Unspecified, correct for use with the framebuffer.
The kernel and the lower-half identity mapping will be mapped with Normal write-back memory,
while the framebuffer is mapped with the correct caching mode. The kernel must ensure that
MMIO it wants to access is mapped with the correct caching mode.
The `MAIR_EL1` register contents are described above, in the caching section.
All interrupts are masked (`PSTATE.{D, A, I, F}` are set to 1).
The kernel is entered in little-endian AArch64 EL1t (EL1 with `PSTATE.SP` set to 0, `PSTATE.E` set to 0, and `PSTATE.nRW` set to 0).
The kernel is entered in little-endian AArch64 EL1t (EL1 with `PSTATE.SP` set to
0, `PSTATE.E` set to 0, and `PSTATE.nRW` set to 0).
Other fields of `PSTATE` are undefined.
At entry: the MMU (`SCTLR_EL1.M`) is enabled, the I-Cache and D-Cache (`SCTLR_EL1.{I, C}`) are enabled,
data alignment checking (`SCTLR_EL1.A`) is disabled. SP alignment checking (`SCTLR_EL1.{SA, SA0}`) is enabled.
Other fields of `SCTLR_EL1` are reset to 0 or to their reserved value.
At entry: the MMU (`SCTLR_EL1.M`) is enabled, the I-Cache and D-Cache
(`SCTLR_EL1.{I, C}`) are enabled, data alignment checking (`SCTLR_EL1.A`) is
disabled. SP alignment checking (`SCTLR_EL1.{SA, SA0}`) is enabled. Other fields
of `SCTLR_EL1` are reset to 0 or to their reserved value.
Higher ELs do not interfere with accesses to vector or floating point instructions or registers.
Higher ELs do not interfere with accesses to vector or floating point
instructions or registers.
Higher ELs do not interfere with accesses to the generic timer and counter.
@ -195,7 +244,8 @@ If booted by EFI/UEFI, boot services are exited.
at least 64KiB (65536 bytes) in size, or the size specified in the Stack
Size Request (see below).
All other general purpose registers (including `X29` and `X30`) are set to 0. Vector registers are in an undefined state.
All other general purpose registers (including `X29` and `X30`) are set to 0.
Vector registers are in an undefined state.
### riscv64