On 5/14/20 11:22 AM, Ard Biesheuvel wrote:
Hi Sami,
On 5/14/20 10:40 AM, Sami Mujawar wrote:
Some device drivers perform copy operations on
device memory, e.g. device drivers for a Flash
device. On some architectures unaligned access
to device memory regions is not permitted. To
add to this if the device is virtualised then
there are further restrictions on the type of
load/store operations that can be performed
on the device memory regions, e.g. on AARCH64,
Pre/Post index or LDP operations cannot be
used, as a trap to EL2 does not provide the
syndrome information to the hypervisor.
We are conflating two different things here:
- the use of unaligned accesses to read from device memory
- the use of load/store instructions that have multiple output
registers, making it very difficult to emulate them in a virtual machine
context.
I though that for the kvmtool port, we addressed the second issue by
using a read-only memslot for the NOR flash when it is in array mode?
IIRC, the issue only affects reads, as all writes to NOR flash device
memory are already done using the proper MMIO accessors. If those MMIO
accessors compile to something that does not work under emulation, you
should be use the special IoLib implementation that exists for this use
case:
MdePkg/Library/BaseIoLibIntrinsic/BaseIoLibIntrinsicArmVirt.inf
So that leaves the issue of unaligned accesses to device memory. This
should not happen unless you are using BaseMemoryLibOptDxe for AARCH64,
which deliberately uses overlapping loads and stores for performance.
Any BASE library is built with -mstrict-align, and so any other C code
implementation of BaseMemoryLib should work fine for reading for a
read-only memslot mapped using device attributes.
So in summary, I don't think we need this library.
BTW I think the AlignedCopyMem() introduction predates the changes to
use -mstrict-align for BASE libraries, and to use the ordinary
BaseMemoryLib implementation for DXE_RUNTIME_DRIVER modules on AArch64.
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