lkml.org 
[lkml]   [2021]   [Sep]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [RFC] KVM: mm: fd-based approach for supporting KVM guest private memory
From
Date
On 31.08.21 22:45, Sean Christopherson wrote:
> On Tue, Aug 31, 2021, David Hildenbrand wrote:
>> On 28.08.21 00:28, Sean Christopherson wrote:
>>> On Fri, Aug 27, 2021, Andy Lutomirski wrote:
>>>>
>>>> On Thu, Aug 26, 2021, at 2:26 PM, David Hildenbrand wrote:
>>>>> On 26.08.21 19:05, Andy Lutomirski wrote:
>>>>
>>>>>> Oof. That's quite a requirement. What's the point of the VMA once all
>>>>>> this is done?
>>>>>
>>>>> You can keep using things like mbind(), madvise(), ... and the GUP code
>>>>> with a special flag might mostly just do what you want. You won't have
>>>>> to reinvent too many wheels on the page fault logic side at least.
>>>
>>> Ya, Kirill's RFC more or less proved a special GUP flag would indeed Just Work.
>>> However, the KVM page fault side of things would require only a handful of small
>>> changes to send private memslots down a different path. Compared to the rest of
>>> the enabling, it's quite minor.
>>>
>>> The counter to that is other KVM architectures would need to learn how to use the
>>> new APIs, though I suspect that there will be a fair bit of arch enabling regardless
>>> of what route we take.
>>>
>>>> You can keep calling the functions. The implementations working is a
>>>> different story: you can't just unmap (pte_numa-style or otherwise) a private
>>>> guest page to quiesce it, move it with memcpy(), and then fault it back in.
>>>
>>> Ya, I brought this up in my earlier reply. Even the initial implementation (without
>>> real NUMA support) would likely be painful, e.g. the KVM TDX RFC/PoC adds dedicated
>>> logic in KVM to handle the case where NUMA balancing zaps a _pinned_ page and then
>>> KVM fault in the same pfn. It's not thaaat ugly, but it's arguably more invasive
>>> to KVM's page fault flows than a new fd-based private memslot scheme.
>>
>> I might have a different mindset, but less code churn doesn't necessarily
>> translate to "better approach".
>
> I wasn't referring to code churn. By "invasive" I mean number of touchpoints in
> KVM as well as the nature of the touchpoints. E.g. poking into how KVM uses
> available bits in its shadow PTEs and adding multiple checks through KVM's page
> fault handler, versus two callbacks to get the PFN and page size.
>
>> I'm certainly not pushing for what I proposed (it's a rough, broken sketch).
>> I'm much rather trying to come up with alternatives that try solving the
>> same issue, handling the identified requirements.
>>
>> I have a gut feeling that the list of requirements might not be complete
>> yet. For example, I wonder if we have to protect against user space
>> replacing private pages by shared pages or punishing random holes into the
>> encrypted memory fd.
>
> Replacing a private page with a shared page for a given GFN is very much a
> requirement as it's expected behavior for all VMM+guests when converting guest
> memory between shared and private.
>
> Punching holes is a sort of optional requirement. It's a "requirement" in that
> it's allowed if the backing store supports such a behavior, optional in that
> support wouldn't be strictly necessary and/or could come with constraints. The
> expected use case is that host userspace would punch a hole to free unreachable
> private memory, e.g. after the corresponding GFN(s) is converted to shared, so
> that it doesn't consume 2x memory for the guest.
>

Okay, that matches my understanding then. I was rather thinking about
"what happens if we punch a hole where private memory was not converted
to shared yet". AFAIU, we will simply crash the guest then.

--
Thanks,

David / dhildenb

\
 
 \ /
  Last update: 2021-09-01 09:52    [W:0.103 / U:1.040 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site