Re: [PATCH rdma-next 1/2] arm64/io: add memcpy_toio_64

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Jan 17, 2024 at 12:30:00PM +0000, Mark Rutland wrote:
> On Tue, Jan 16, 2024 at 02:51:21PM -0400, Jason Gunthorpe wrote:
> > Hey Catalin,
> > 
> > I'm just revising this and I'm wondering if you know why ARM64 has this:
> > 
> > #define __raw_writeq __raw_writeq
> > static __always_inline void __raw_writeq(u64 val, volatile void __iomem *addr)
> > {
> > 	asm volatile("str %x0, [%1]" : : "rZ" (val), "r" (addr));
> > }
> > 
> > Instead of
> > 
> > #define __raw_writeq __raw_writeq
> > static __always_inline void __raw_writeq(u64 val, volatile void __iomem *addr)
> > {
> > 	asm volatile("str %x0, %1" : : "rZ" (val), "m" (*(volatile u64 *)addr));
> > }
> > 
> > ?? Like x86 has.
> 
> I believe this is for the same reason as doing so in all of our other IO
> accessors.
> 
> We've deliberately ensured that our IO accessors use a single base register
> with no offset as this is the only form that HW can represent in ESR_ELx.ISS.SRT
> when reporting a stage-2 abort, which a hypervisor may use for
> emulating IO.

Wow, harming bare metal performace to accommodate imperfect emulation
sounds like a horrible reason :(

So what happens with this patch where IO is done with STP? Are you
going to tell me I can't do it because of this?

Jason