Re: [PATCH RFC 0/9] PKS write protected page tables

Peter Zijlstra
2021-05-05T11:58:03Z

On Wed, May 05, 2021 at 01:08:35PM +0200, Vlastimil Babka wrote:
> On 5/5/21 2:30 AM, Rick Edgecombe wrote:

> > Why use PKS for this?
> > =====================
> > PKS is an upcoming CPU feature that allows supervisor virtual memory
> > permissions to be changed without flushing the TLB, like PKU does for user
> > memory. Protecting page tables would normally be really expensive because you
> > would have to do it with paging itself. PKS helps by providing a way to toggle
> > the writability of the page tables with just a per-cpu MSR.
>
> I can see in patch 8/9 that you are flipping the MSR around individual
> operations on page table entries. In my patch I hooked making the page table
> writable to obtaining the page table lock (IIRC I had only the PTE level fully
> handled though). Wonder if that would be better tradeoff even for your MSR approach?

There's also the HIGHPTE code we could abuse to kmap an alias while
we're at it.