Re: [PATCH Part2 v5 39/45] KVM: SVM: Introduce ops for the post gfn map and unmap

Brijesh Singh
2021-10-13T21:49:35Z

On 10/13/21 1:10 PM, Sean Christopherson wrote:
> On Wed, Oct 13, 2021, Brijesh Singh wrote:
>> On 10/12/21 5:23 PM, Sean Christopherson wrote:
>>> On Fri, Aug 20, 2021, Brijesh Singh wrote:
>>>> When SEV-SNP is enabled in the guest VM, the guest memory pages can
>>>> either be a private or shared. A write from the hypervisor goes through
>>>> the RMP checks. If hardware sees that hypervisor is attempting to write
>>>> to a guest private page, then it triggers an RMP violation #PF.
>>>>
>>>> To avoid the RMP violation, add post_{map,unmap}_gfn() ops that can be
>>>> used to verify that its safe to map a given guest page. Use the SRCU to
>>>> protect against the page state change for existing mapped pages.
>>> SRCU isn't protecting anything. The synchronize_srcu_expedited() in the PSC code
>>> forces it to wait for existing maps to go away, but it doesn't prevent new maps
>>> from being created while the actual RMP updates are in-flight. Most telling is
>>> that the RMP updates happen _after_ the synchronize_srcu_expedited() call.
>>>
>>> This also doesn't handle kvm_{read,write}_guest_cached().
>> Hmm, I thought the kvm_{read_write}_guest_cached() uses the
>> copy_{to,from}_user(). Writing to the private will cause a #PF and will
>> fail the copy_to_user(). Am I missing something?
> Doh, right you are. I was thinking they cached the kmap, but it's just the
> gpa->hva that gets cached.
>
>>> I can't help but note that the private memslots idea[*] would handle this gracefully,
>>> e.g. the memslot lookup would fail, and any change in private memslots would
>>> invalidate the cache due to a generation mismatch.
>>>
>>> KSM is another mess that would Just Work.
>>>
>>> I'm not saying that SNP should be blocked on support for unmapping guest private
>>> memory, but I do think we should strongly consider focusing on that effort rather
>>> than trying to fix things piecemeal throughout KVM. I don't think it's too absurd
>>> to say that it might actually be faster overall. And I 100% think that having a
>>> cohesive design and uABI for SNP and TDX would be hugely beneficial to KVM.
>>>
>>> [*] https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flkml.kernel.org%2Fr%2F20210824005248.200037-1-seanjc%40google.com&amp;data=04%7C01%7Cbrijesh.singh%40amd.com%7C0f1a3f5f63074b60d21b08d98e857daf%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637697526304105177%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&amp;sdata=w6xS3DcG4fcTweC5i4%2BuB4jhn3Xcj2a44BkoATVcSgQ%3D&amp;reserved=0
>>>
>>>> +int sev_post_map_gfn(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, int *token)
>>>> +{
>>>> + struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
>>>> + int level;
>>>> +
>>>> + if (!sev_snp_guest(kvm))
>>>> + return 0;
>>>> +
>>>> + *token = srcu_read_lock(&sev->psc_srcu);
>>>> +
>>>> + /* If pfn is not added as private then fail */
>>> This comment and the pr_err() are backwards, and confused the heck out of me.
>>> snp_lookup_rmpentry() returns '1' if the pfn is assigned, a.k.a. private. That
>>> means this code throws an error if the page is private, i.e. requires the page
>>> to be shared. Which makes sense given the use cases, it's just incredibly
>>> confusing.
>> Actually I followed your recommendation from the previous feedback that
>> snp_lookup_rmpentry() should return 1 for the assigned page, 0 for the
>> shared and -negative for invalid. I can clarify it here  again.
>>
>>>> + if (snp_lookup_rmpentry(pfn, &level) == 1) {
>>> Any reason not to provide e.g. rmp_is_shared() and rmp_is_private() so that
>>> callers don't have to care as much about the return values? The -errno/0/1
>>> semantics are all but guarantee to bite us in the rear at some point.
>> If we look at the previous series, I had a macro rmp_is_assigned() for
>> exactly the same purpose but the feedback was to drop those macros and
>> return the state indirectly through the snp_lookup_rmpentry(). I can
>> certainly add a helper rmp_is_{shared,private}() if it makes code more
>> readable.
> Ah rats. Bad communication on my side. I didn't intended to have non-RMP code
> directly consume snp_lookup_rmpentry() for simple checks. The goal behind the
> helper was to bury "struct rmpentry" so that it wasn't visible to the kernel at
> large. I.e. my objection was that rmp_assigned() was looking directly at a
> non-architectural struct.
>
> My full thought for snp_lookup_rmpentry() was that it _could_ be consumed directly
> without exposing "struct rmpentry", but that simple, common use cases would provide
> wrappers, e.g.
>
> static inline snp_is_rmp_private(...)
> {
> return snp_lookup_rmpentry(...) == 1;
> }
>
> static inline snp_is_rmp_shared(...)
> {
> return snp_lookup_rmpentry(...) < 1;
> }

Yep, that what I was going to do for the helper.

>
> Side topic, what do you think about s/assigned/private for the "public" APIs, as
> suggested/implied above? I actually like the terminology when talking specifically
> about the RMP, but it doesn't fit the abstractions that tend to be used when talking
> about these things in other contexts, e.g. in KVM.

I can float the idea to see if docs folks is okay with the changes but
generally speaking we all have been referring the assigned == private in
the Linux SNP support patch.

thanks