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

From: Nadav Amit
Date: Mon Jun 17 2019 - 14:55:36 EST

> On Jun 17, 2019, at 11:07 AM, Dave Hansen <dave.hansen@xxxxxxxxx> 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.

I understand, but as I see it, this is not related directly to PCIDs.