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

From: Konrad Rzeszutek Wilk
Date: Mon Jun 17 2019 - 14:50:01 EST

On Mon, Jun 17, 2019 at 11:07:45AM -0700, Dave Hansen wrote:
> 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.

But you would need to invalidate if the process moved to another CPU, correct?