Re: [PATCH TEST-ARTEFACTS 0/8] Cleanup and Linux ARM64 support

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

On Thu, 2025-04-10 at 19:28 +0100, Alejandro Vallejo wrote:
> The cutoff point is bcf0e2fda80c6("x86/xen: remove hypercall page").
> 
> Together with Jason's observation it would seem that Linux doesn't guess
> the correct instruction (or not early enough) when running as PVH dom0.
> On PV it's just "syscall", but on PVH it's a tad more complicated.
> 

I never understood why we did it this way anyway.

All this bogus complexity to do early detection of AMD vs. Intel and
use the right trap instruction, when we could have just continued to
use the proper Xen hypercall page at early boot.

After all, if you don't know what kind of CPU you're on, you *also*
haven't enabled CET or any other fancy return-tracking stuff yet. Just
fill in your own hypercall page *then*, instead. And then you can free
the original one.

Much simpler, much less fragile, and less prone to other potential
breakage from 64-bit latching side-effect that we forgot Xen does when
the guest sets the hypercall page.

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