* 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
