Re: [RFC PATCH v3 13/15] context_tracking,x86: Add infrastructure to defer kernel TLBI

On Wed, Nov 20, 2024 at 04:22:16PM +0100, Peter Zijlstra wrote:

On Tue, Nov 19, 2024 at 04:35:00PM +0100, Valentin Schneider wrote:

...

+void noinstr __flush_tlb_all_noinstr(void) +{

• /*

• * This is for invocation in early entry code that cannot be
  

• * instrumented. A RMW to CR4 works for most cases, but relies on
  

• * being able to flip either of the PGE or PCIDE bits. Flipping CR4.PCID
  

• * would require also resetting CR3.PCID, so just try with CR4.PGE, else
  

• * do the CR3 write.
  

• *
  

• * XXX: this gives paravirt the finger.
  

• */
  

• if (cpu_feature_enabled(X86_FEATURE_PGE))

• __native_tlb_flush_global_noinstr(this_cpu_read(cpu_tlbstate.cr4));
  

• else

• native_flush_tlb_local_noinstr();
  

+}

Urgh, so that's a lot of ugleh, and cr4 has that pinning stuff and gah.

Why not always just do the CR3 write and call it a day? That should also work for paravirt, no? Just make the whole write_cr3 thing noinstr and voila.

Oh gawd, just having looked at xen_write_cr3() this might not be entirely trivial to mark noinstr :/