Re: [PATCH v2 1/9] KVM: x86: Add AMD SEV specific Hypercall3

[Dr. David Alan Gilbert]

* Kalra, Ashish (Ashish.Kalra@xxxxxxx) wrote:
> Hello Dave,
> 
> On Dec 18, 2020, at 1:40 PM, Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx> wrote:
> 
> * Ashish Kalra (ashish.kalra@xxxxxxx) wrote:
> On Fri, Dec 11, 2020 at 10:55:42PM +0000, Ashish Kalra wrote:
> Hello All,
> 
> On Tue, Dec 08, 2020 at 10:29:05AM -0600, Brijesh Singh wrote:
> 
> On 12/7/20 9:09 PM, Steve Rutherford wrote:
> On Mon, Dec 7, 2020 at 12:42 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> On Sun, Dec 06, 2020, Paolo Bonzini wrote:
> On 03/12/20 01:34, Sean Christopherson wrote:
> On Tue, Dec 01, 2020, Ashish Kalra wrote:
> From: Brijesh Singh <brijesh.singh@xxxxxxx>
> 
> KVM hypercall framework relies on alternative framework to patch the
> VMCALL -> VMMCALL on AMD platform. If a hypercall is made before
> apply_alternative() is called then it defaults to VMCALL. The approach
> works fine on non SEV guest. A VMCALL would causes #UD, and hypervisor
> will be able to decode the instruction and do the right things. But
> when SEV is active, guest memory is encrypted with guest key and
> hypervisor will not be able to decode the instruction bytes.
> 
> Add SEV specific hypercall3, it unconditionally uses VMMCALL. The hypercall
> will be used by the SEV guest to notify encrypted pages to the hypervisor.
> What if we invert KVM_HYPERCALL and X86_FEATURE_VMMCALL to default to VMMCALL
> and opt into VMCALL?  It's a synthetic feature flag either way, and I don't
> think there are any existing KVM hypercalls that happen before alternatives are
> patched, i.e. it'll be a nop for sane kernel builds.
> 
> I'm also skeptical that a KVM specific hypercall is the right approach for the
> encryption behavior, but I'll take that up in the patches later in the series.
> Do you think that it's the guest that should "donate" memory for the bitmap
> instead?
> No.  Two things I'd like to explore:
> 
>  1. Making the hypercall to announce/request private vs. shared common across
>     hypervisors (KVM, Hyper-V, VMware, etc...) and technologies (SEV-* and TDX).
>     I'm concerned that we'll end up with multiple hypercalls that do more or
>     less the same thing, e.g. KVM+SEV, Hyper-V+SEV, TDX, etc...  Maybe it's a
>     pipe dream, but I'd like to at least explore options before shoving in KVM-
>     only hypercalls.
> 
> 
>  2. Tracking shared memory via a list of ranges instead of a using bitmap to
>     track all of guest memory.  For most use cases, the vast majority of guest
>     memory will be private, most ranges will be 2mb+, and conversions between
>     private and shared will be uncommon events, i.e. the overhead to walk and
>     split/merge list entries is hopefully not a big concern.  I suspect a list
>     would consume far less memory, hopefully without impacting performance.
> For a fancier data structure, I'd suggest an interval tree. Linux
> already has an rbtree-based interval tree implementation, which would
> likely work, and would probably assuage any performance concerns.
> 
> Something like this would not be worth doing unless most of the shared
> pages were physically contiguous. A sample Ubuntu 20.04 VM on GCP had
> 60ish discontiguous shared regions. This is by no means a thorough
> search, but it's suggestive. If this is typical, then the bitmap would
> be far less efficient than most any interval-based data structure.
> 
> You'd have to allow userspace to upper bound the number of intervals
> (similar to the maximum bitmap size), to prevent host OOMs due to
> malicious guests. There's something nice about the guest donating
> memory for this, since that would eliminate the OOM risk.
> 
> 
> Tracking the list of ranges may not be bad idea, especially if we use
> the some kind of rbtree-based data structure to update the ranges. It
> will certainly be better than bitmap which grows based on the guest
> memory size and as you guys see in the practice most of the pages will
> be guest private. I am not sure if guest donating a memory will cover
> all the cases, e.g what if we do a memory hotplug (increase the guest
> ram from 2GB to 64GB), will donated memory range will be enough to store
> the metadata.
> 
> .
> 
> With reference to internal discussions regarding the above, i am going
> to look into specific items as listed below :
> 
> 1). "hypercall" related :
> a). Explore the SEV-SNP page change request structure (included in GHCB),
> see if there is something common there than can be re-used for SEV/SEV-ES
> page encryption status hypercalls.
> b). Explore if there is any common hypercall framework i can use in
> Linux/KVM.
> 
> 2). related to the "backing" data structure - explore using a range-based
> list or something like rbtree-based interval tree data structure
> (as mentioned by Steve above) to replace the current bitmap based
> implementation.
> 
> 
> 
> I do agree that a range-based list or an interval tree data structure is a
> really good "logical" fit for the guest page encryption status tracking.
> 
> We can only keep track of the guest unencrypted shared pages in the
> range(s) list (which will keep the data structure quite compact) and all
> the guest private/encrypted memory does not really need any tracking in
> the list, anything not in the list will be encrypted/private.
> 
> Also looking at a more "practical" use case, here is the current log of
> page encryption status hypercalls when booting a linux guest :
> 
> ...
> 
> <snip>
> 
> [   56.146336] page_enc_status_hc invoked, gpa = 1f018000, npages  = 1, enc = 1
> [   56.146351] page_enc_status_hc invoked, gpa = 1f00e000, npages  = 1, enc = 0
> [   56.147261] page_enc_status_hc invoked, gpa = 1f00e000, npages  = 1, enc = 0
> [   56.147271] page_enc_status_hc invoked, gpa = 1f018000, npages  = 1, enc = 0
> ....
> 
> [   56.180730] page_enc_status_hc invoked, gpa = 1f008000, npages  = 1, enc = 0
> [   56.180741] page_enc_status_hc invoked, gpa = 1f006000, npages  = 1, enc = 0
> [   56.180768] page_enc_status_hc invoked, gpa = 1f008000, npages  = 1, enc = 1
> [   56.180782] page_enc_status_hc invoked, gpa = 1f006000, npages  = 1, enc = 1
> 
> ....
> [   56.197110] page_enc_status_hc invoked, gpa = 1f007000, npages  = 1, enc = 0
> [   56.197120] page_enc_status_hc invoked, gpa = 1f005000, npages  = 1, enc = 0
> [   56.197136] page_enc_status_hc invoked, gpa = 1f007000, npages  = 1, enc = 1
> [   56.197148] page_enc_status_hc invoked, gpa = 1f005000, npages  = 1, enc = 1
> ....
> 
> [   56.222679] page_enc_status_hc invoked, gpa = 1e83b000, npages  = 1, enc = 0
> [   56.222691] page_enc_status_hc invoked, gpa = 1e839000, npages  = 1, enc = 0
> [   56.222707] page_enc_status_hc invoked, gpa = 1e83b000, npages  = 1, enc = 1
> [   56.222720] page_enc_status_hc invoked, gpa = 1e839000, npages  = 1, enc = 1
> ....
> 
> [   56.313747] page_enc_status_hc invoked, gpa = 1e5eb000, npages  = 1, enc = 0
> [   56.313771] page_enc_status_hc invoked, gpa = 1e5e9000, npages  = 1, enc = 0
> [   56.313789] page_enc_status_hc invoked, gpa = 1e5eb000, npages  = 1, enc = 1
> [   56.313803] page_enc_status_hc invoked, gpa = 1e5e9000, npages  = 1, enc = 1
> ....
> [   56.459276] page_enc_status_hc invoked, gpa = 1d767000, npages  = 100, enc = 0
> [   56.459428] page_enc_status_hc invoked, gpa = 1e501000, npages  = 1, enc = 1
> [   56.460037] page_enc_status_hc invoked, gpa = 1d767000, npages  = 100, enc = 1
> [   56.460216] page_enc_status_hc invoked, gpa = 1e501000, npages  = 1, enc = 0
> [   56.460299] page_enc_status_hc invoked, gpa = 1d767000, npages  = 100, enc = 0
> [   56.460448] page_enc_status_hc invoked, gpa = 1e501000, npages  = 1, enc = 1
> ....
> 
> As can be observed here, all guest MMIO ranges are initially setup as
> shared, and those are all contigious guest page ranges.
> 
> After that the encryption status hypercalls are invoked when DMA gets
> triggered during disk i/o while booting the guest ... here again the
> guest page ranges are contigious, though mostly single page is touched
> and a lot of page re-use is observed.
> 
> So a range-based list/structure will be a "good" fit for such usage
> scenarios.
> 
> It seems surprisingly common to flick the same pages back and forth between
> encrypted and clear for quite a while;  why is this?
> 
> 
> dma_alloc_coherent()'s will allocate pages and then call
> set_decrypted() on them and then at dma_free_coherent(), set_encrypted()
> is called on the pages to be freed. So these observations in the logs
> where a lot of single 4K pages are seeing C-bit transitions and
> corresponding hypercalls are the ones associated with
> dma_alloc_coherent().

It makes me wonder if it might be worth teaching it to hold onto those
DMA pages somewhere until it needs them for something else and avoid the
extra hypercalls; just something to think about.

Dave

> Thanks,
> Ashish
> 
> 
> --
> Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
> 
-- 
Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK