Re: [PATCH v4 26/30] x86,tlb: Make __flush_tlb_global() noinstr-compliant

[Dave Hansen <dave.hansen@xxxxxxxxx>]

On 1/14/25 09:51, Valentin Schneider wrote:
> +     cr4 = this_cpu_read(cpu_tlbstate.cr4);
> +     asm volatile("mov %0,%%cr4": : "r" (cr4 ^ X86_CR4_PGE) : "memory");
> +     asm volatile("mov %0,%%cr4": : "r" (cr4) : "memory");
> +     /*
> +      * In lieu of not having the pinning crap, hard fail if CR4 doesn't
> +      * match the expected value. This ensures that anybody doing dodgy gets
> +      * the fallthrough check.
> +      */
> +     BUG_ON(cr4 != this_cpu_read(cpu_tlbstate.cr4));

Let's say someone managed to write to cpu_tlbstate.cr4 where they
cleared one of the pinned bits.

Before this patch, CR4 pinning would WARN_ONCE() about it pretty quickly
and also reset the cleared bits.

After this patch, the first native_flush_tlb_global() can clear pinned
bits, at least until native_write_cr4() gets called the next time. That
seems like it'll undermine CR4 pinning at least somewhat.

What keeps native_write_cr4() from being noinstr-compliant now? Is it
just the WARN_ONCE()?

If so, I'd kinda rather have a native_write_cr4_nowarn() that's
noinstr-compliant but retains all the other CR4 pinning behavior. Would
something like the attached patch be _worse_?

Attachment: cr4.patch
Description: Text Data