Re: [PATCH v11 5/9] x86/efi: Disable LASS while mapping the EFI runtime services

Dave Hansen
2025-10-31T17:11:42Z

On 10/29/25 14:03, Sohil Mehta wrote:
> From: Alexander Shishkin <alexander.shishkin@linux.intel.com>
>
> While mapping EFI runtime services, set_virtual_address_map() is called
> at its lower mapping, which LASS prohibits. Wrapping the EFI call with
> lass_disable()/_enable() is not enough, because the AC flag only
> controls data accesses, and not instruction fetches.
>
> Use the big hammer and toggle the CR4.LASS bit to make this work.

One thing that's actually missing here is an explanation on how it's OK
to munge CR bits here. Why are preemption and interrupts not a problem?

A reviewer would have to go off and figure this out on their own.