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Thu, 08 Aug 2024 22:16:11 +0000 (GMT) Received: from nasanex01b.na.qualcomm.com (nasanex01b.na.qualcomm.com [10.46.141.250]) by NASANPPMTA02.qualcomm.com (8.17.1.19/8.17.1.19) with ESMTPS id 478MGA6T016945 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Thu, 8 Aug 2024 22:16:10 GMT Received: from hu-eberman-lv.qualcomm.com (10.49.16.6) by nasanex01b.na.qualcomm.com (10.46.141.250) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1544.9; Thu, 8 Aug 2024 15:16:09 -0700 Date: Thu, 8 Aug 2024 15:16:09 -0700 From: Elliot Berman To: Patrick Roy CC: Andrew Morton , Paolo Bonzini , Sean Christopherson , Fuad Tabba , David Hildenbrand , , Ackerley Tng , , , , , , James Gowans , "Kalyazin, Nikita" , "Manwaring, Derek" , "Cali, Marco" Subject: Re: [PATCH RFC 3/4] mm: guest_memfd: Add option to remove guest private memory from direct map Message-ID: <20240808145103617-0700.eberman@hu-eberman-lv.qualcomm.com> References: <20240805-guest-memfd-lib-v1-0-e5a29a4ff5d7@quicinc.com> <20240805-guest-memfd-lib-v1-3-e5a29a4ff5d7@quicinc.com> <3fc11402-53e1-4325-a3ee-5ebd616b5b63@amazon.co.uk> <20240806104702482-0700.eberman@hu-eberman-lv.qualcomm.com> <90886a03-ad62-4e98-bc05-63875faa9ccc@amazon.co.uk> <20240807113514068-0700.eberman@hu-eberman-lv.qualcomm.com> <7166d51c-7757-44f2-a6f8-36da3e86bf90@amazon.co.uk> Precedence: bulk X-Mailing-List: linux-coco@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <7166d51c-7757-44f2-a6f8-36da3e86bf90@amazon.co.uk> X-ClientProxiedBy: nalasex01b.na.qualcomm.com (10.47.209.197) To nasanex01b.na.qualcomm.com (10.46.141.250) X-QCInternal: smtphost X-Proofpoint-Virus-Version: vendor=nai engine=6200 definitions=5800 signatures=585085 X-Proofpoint-GUID: BV9dcG3O0hDNp8CTu0jKw-Sde_t8cyjh X-Proofpoint-ORIG-GUID: BV9dcG3O0hDNp8CTu0jKw-Sde_t8cyjh X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1039,Hydra:6.0.680,FMLib:17.12.28.16 definitions=2024-08-08_22,2024-08-07_01,2024-05-17_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 suspectscore=0 malwarescore=0 adultscore=0 clxscore=1015 priorityscore=1501 lowpriorityscore=0 impostorscore=0 spamscore=0 phishscore=0 bulkscore=0 mlxlogscore=999 mlxscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2407110000 definitions=main-2408080159 On Thu, Aug 08, 2024 at 02:05:55PM +0100, Patrick Roy wrote: > On Wed, 2024-08-07 at 20:06 +0100, Elliot Berman wrote: > >>>>>> struct folio *guest_memfd_grab_folio(struct file *file, pgoff_t index, u32 flags) > >>>>>> { > >>>>>> + unsigned long gmem_flags = (unsigned long)file->private_data; > >>>>>> struct inode *inode = file_inode(file); > >>>>>> struct guest_memfd_operations *ops = inode->i_private; > >>>>>> struct folio *folio; > >>>>>> @@ -43,6 +89,12 @@ struct folio *guest_memfd_grab_folio(struct file *file, pgoff_t index, u32 flags > >>>>>> goto out_err; > >>>>>> } > >>>>>> > >>>>>> + if (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP) { > >>>>>> + r = guest_memfd_folio_private(folio); > >>>>>> + if (r) > >>>>>> + goto out_err; > >>>>>> + } > >>>>>> + > >>>>> > >>>>> How does a caller of guest_memfd_grab_folio know whether a folio needs > >>>>> to be removed from the direct map? E.g. how can a caller know ahead of > >>>>> time whether guest_memfd_grab_folio will return a freshly allocated > >>>>> folio (which thus needs to be removed from the direct map), vs a folio > >>>>> that already exists and has been removed from the direct map (probably > >>>>> fine to remove from direct map again), vs a folio that already exists > >>>>> and is currently re-inserted into the direct map for whatever reason > >>>>> (must not remove these from the direct map, as other parts of > >>>>> KVM/userspace probably don't expect the direct map entries to disappear > >>>>> from underneath them). I couldn't figure this one out for my series, > >>>>> which is why I went with hooking into the PG_uptodate logic to always > >>>>> remove direct map entries on freshly allocated folios. > >>>>> > >>>> > >>>> gmem_flags come from the owner. If the caller (in non-CoCo case) wants > >> > >> Ah, oops, I got it mixed up with the new `flags` parameter. > >> > >>>> to restore the direct map right away, it'd have to be a direct > >>>> operation. As an optimization, we could add option that asks for page in > >>>> "shared" state. If allocating new page, we can return it right away > >>>> without removing from direct map. If grabbing existing folio, it would > >>>> try to do the private->shared conversion. > >> > >> My concern is more with the implicit shared->private conversion that > >> happens on every call to guest_memfd_grab_folio (and thus > >> kvm_gmem_get_pfn) when grabbing existing folios. If something else > >> marked the folio as shared, then we cannot punch it out of the direct > >> map again until that something is done using the folio (when working on > >> my RFC, kvm_gmem_get_pfn was indeed called on existing folios that were > >> temporarily marked shared, as I was seeing panics because of this). And > >> if the folio is currently private, there's nothing to do. So either way, > >> guest_memfd_grab_folio shouldn't touch the direct map entry for existing > >> folios. > >> > > > > What I did could be documented/commented better. > > No worries, thanks for taking the time to walk me through understanding > it! > > > If ops->accessible() is *not* provided, all guest_memfd allocations will > > immediately remove from direct map and treat them immediately like guest > > private (goal is to match what KVM does today on tip). > > Ah, so if ops->accessible() is not provided, then there will never be > any shared memory inside gmem (like today, where gmem doesn't support > shared memory altogether), and thus there's no problems with just > unconditionally doing set_direct_map_invalid_noflush in > guest_memfd_grab_folio, because all existing folios already have their > direct map entry removed. Got it! > > > If ops->accessible() is provided, then guest_memfd allocations start > > as "shared" and KVM/Gunyah need to do the shared->private conversion > > when they want to do the private conversion on the folio. "Shared" is > > the default because that is effectively a no-op. > > For the non-CoCo case you're interested in, we'd have the > > ops->accessible() provided and we wouldn't pull out the direct map from > > gpc. > > So in pKVM/Gunyah's case, guest memory starts as shared, and at some > point the guest will issue a hypercall (or similar) to flip it to > private, at which point it'll get removed from the direct map? > > That isn't really what we want for our case. We consider the folios as > private straight away, as we do not let the guest control their state at > all. Everything is always "accessible" to both KVM and userspace in the > sense that they can just flip gfns to shared as they please without the > guest having any say in it. > > I think we should untangle the behavior of guest_memfd_grab_folio from > the presence of ops->accessible. E.g. instead of direct map removal > being dependent on ops->accessible we should have some > GRAB_FOLIO_RETURN_SHARED flag for gmem_flags, which is set for y'all, > and not set for us (I don't think we should have a "call > set_direct_map_invalid_noflush unconditionally in > guest_memfd_grab_folio" mode at all, because if sharing gmem is > supported, then that is broken, and if sharing gmem is not supported > then only removing direct map entries for freshly allocated folios gets > us the same result of "all folios never in the direct map" while > avoiding some no-op direct map operations). > > Because we would still use ->accessible, albeit for us that would be > more for bookkeeping along the lines of "which gfns does userspace > currently require to be in the direct map?". I haven't completely > thought it through, but what I could see working for us would be a pair > of ioctls for marking ranges accessible/inaccessible, with > "accessibility" stored in some xarray (somewhat like Fuad's patches, I > guess? [1]). > > In a world where we have a "sharing refcount", the "make accessible" > ioctl reinserts into the direct map (if needed), lifts the "sharings > refcount" for each folio in the given gfn range, and marks the range as > accessible. And the "make inaccessible" ioctl would first check that > userspace has unmapped all those gfns again, and if yes, mark them as > inaccessible, drop the "sharings refcount" by 1 for each, and removes > from the direct map again if it held the last reference (if userspace > still has some gfns mapped, the ioctl would just fail). > I am warming up to the sharing refcount idea. How does the sharing refcount look for kvm gpc? > I guess for pKVM/Gunyah, there wouldn't be userspace ioctls, but instead > the above would happen in handlers for share/unshare hypercalls. But the > overall flow would be similar. The only difference is the default state > of guest memory (shared for you, private for us). You want a > guest_memfd_grab_folio that essentially returns folios with "sharing > refcount == 1" (and thus present in the direct map), while we want the > opposite. > > So I think something like the following should work for both of us > (modulo some error handling): > > static struct folio *__kvm_gmem_get_folio(struct file *file, pgoff_t index, bool prepare, bool *fresh) > { > // as today's kvm_gmem_get_folio, except > ... > if (!folio_test_uptodate(folio)) { > ... > if (fresh) > *fresh = true > } > ... > } > > struct folio *kvm_gmem_get_folio(struct file *file, pgoff_t index, bool prepare) > { > bool fresh; > unsigned long gmem_flags = /* ... */ > struct folio *folio = __kvm_gmem_get_folio(file, index, prepare, &fresh); > if (gmem_flag & GRAB_FOLIO_RETURN_SHARED != 0) { > // if "sharing refcount == 0", inserts back into direct map and lifts refcount, otherwise just lifts refcount > guest_memfd_folio_clear_private(folio); > } else { > if (fresh) > guest_memfd_folio_private(folio); > } > return folio; > } > > Now, thinking ahead, there's probably optimizations here where we defer > the direct map manipulations to gmem_fault, at which point having a > guest_memfd_grab_folio that doesn't remove direct map entries for fresh > folios would be useful in our non-CoCo usecase too. But that should also > be easily achievable by maybe having a flag to kvm_gmem_get_folio that > forces the behavior of GRAB_FOLIO_RETURN_SHARED, indendently of whether > GRAB_FOLIO_RETURN_SHARED is set in gmem_flags. > > How does that sound to you? > Yeah, I think this is a good idea. I'm also thinking to make a few tweaks to the ops structure: struct guest_memfd_operations { int (*invalidate_begin)(struct inode *inode, pgoff_t offset, unsigned long nr); void (*invalidate_end)(struct inode *inode, pgoff_t offset, unsigned long nr); int (*prepare_accessible)(struct inode *inode, struct folio *folio); int (*prepare_private)(struct inode *inode, struct folio *folio); int (*release)(struct inode *inode); }; When grabbing a folio, we'd always call either prepare_accessible() or prepare_private() based on GRAB_FOLIO_RETURN_SHARED. In the prepare_private() case, guest_memfd can also ensure the folio is unmapped and not pinned. If userspace tries to grab the folio in pKVM/Gunyah case, prepare_accessible() will fail and grab_folio returns error. There's a lot of details I'm glossing over, but I hope it gives some brief idea of the direction I was thinking. In some cases, prepare_accessible() and the invalidate_*() functions might effectively be the same thing, except that invalidate_*() could operate on a range larger-than-a-folio. That would be useful becase we might offer optimization to reclaim a batch of pages versus e.g. flushing caches every page. Thanks, Elliot