mirror of https://github.com/acidanthera/audk.git
fb8b54694c
* Short description:
The CpuIoServiceRead() and CpuIoServiceWrite() functions transfer data
between memory and IO ports with individual Io(Read|Write)(8|16|32)
function calls, each in an appropriately set up loop.
On the Ia32 and X64 platforms however, FIFO reads and writes can be
optimized, by coding them in assembly, and delegating the loop to the
CPU, with the REP prefix.
On KVM virtualization hosts, this difference has a huge performance
impact: if the loop is open-coded, then the virtual machine traps to the
hypervisor on every single UINT8 / UINT16 / UINT32 transfer, whereas
with the REP prefix, KVM can transfer up to a page of data per VM trap.
This is especially noticeable with IDE PIO transfers, where all the data
are squeezed through IO ports.
* Long description:
The RootBridgeIoIoRW() function in
PcAtChipsetPkg/PciHostBridgeDxe/PciRootBridgeIo.c
used to have the exact same IO port acces optimization, dating back
verbatim to commit 1fd376d9792:
PcAtChipsetPkg/PciHostBridgeDxe: Improve KVM FIFO I/O read/write
performance
OvmfPkg cloned the "PcAtChipsetPkg/PciHostBridgeDxe" driver (for
unrelated reasons), and inherited the optimization from PcAtChipsetPkg.
The "PcAtChipsetPkg/PciHostBridgeDxe" driver was ultimately removed in
commit 111d79db47:
PcAtChipsetPkg/PciHostBridge: Remove PciHostBridge driver
and OvmfPkg too was rebased to the new core Pci Host Bridge Driver, in
commit 4014885ffd:
OvmfPkg: switch to MdeModulePkg/Bus/Pci/PciHostBridgeDxe
This caused the optimization to go lost. Namely, the
RootBridgeIoIoRead() and RootBridgeIoIoWrite() functions in the new core
Pci Host Bridge Driver delegate IO port accesses to
EFI_CPU_IO2_PROTOCOL. And, in OvmfPkg (and likely most other Ia32 / X64
edk2 platforms), this protocol is provided by "UefiCpuPkg/CpuIo2Dxe",
which lacks the optimization.
Therefore, this patch ports the C source code logic from commit
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