Re: [RFC 00/10] Process-local memory allocations for hiding KVM secrets

From: Dave Hansen
Date: Mon Jun 17 2019 - 14:19:37 EST

On 6/17/19 9:53 AM, Nadav Amit wrote:
>>> For anyone following along at home, I'm going to go off into crazy
>>> per-cpu-pgds speculation mode now... Feel free to stop reading now. :)
>>> But, I was thinking we could get away with not doing this on _every_
>>> context switch at least. For instance, couldn't 'struct tlb_context'
>>> have PGD pointer (or two with PTI) in addition to the TLB info? That
>>> way we only do the copying when we change the context. Or does that tie
>>> the implementation up too much with PCIDs?
>> Hmm, that seems entirely reasonable. I think the nasty bit would be
>> figuring out all the interactions with PV TLB flushing. PV TLB
>> flushes already don't play so well with PCID tracking, and this will
>> make it worse. We probably need to rewrite all that code regardless.
> How is PCID (as you implemented) related to TLB flushing of kernel (not
> user) PTEs? These kernel PTEs would be global, so they would be invalidated
> from all the address-spaces using INVLPG, I presume. No?

The idea is that you have a per-cpu address space. Certain kernel
virtual addresses would map to different physical address based on where
you are running. Each of the physical addresses would be "owned" by a
single CPU and would, by convention, never use a PGD that mapped an
address unless that CPU that "owned" it.

In that case, you never really invalidate those addresses.