Re: [PATCH v16 1/5] arm/vpci: honor access size when returning an error

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, May 22, 2024 at 06:59:20PM -0400, Stewart Hildebrand wrote:
> From: Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
> 
> Guest can try to read config space using different access sizes: 8,
> 16, 32, 64 bits. We need to take this into account when we are
> returning an error back to MMIO handler, otherwise it is possible to
> provide more data than requested: i.e. guest issues LDRB instruction
> to read one byte, but we are writing 0xFFFFFFFFFFFFFFFF in the target
> register.

Shouldn't this be taken care of in the trap handler subsystem, rather
than forcing each handler to ensure the returned data matches the
access size?

IOW, something like:

diff --git a/xen/arch/arm/io.c b/xen/arch/arm/io.c
index 96c740d5636c..b7e12df85f87 100644
--- a/xen/arch/arm/io.c
+++ b/xen/arch/arm/io.c
@@ -37,6 +37,7 @@ static enum io_state handle_read(const struct mmio_handler *handler,
         return IO_ABORT;

     r = sign_extend(dabt, r);
+    r = r & GENMASK_ULL((1U << dabt.size) * 8 - 1, 0);

     set_user_reg(regs, dabt.reg, r);

(not even build tested)

Thanks, Roger.