From: Sean Christopherson <seanjc@google.com>
To: Kai Huang <kai.huang@intel.com>
Cc: Xiaoyao Li <xiaoyao.li@intel.com>,
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Subject: Re: [PATCH v13 09/35] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report faults to userspace
Date: Thu, 2 Nov 2023 08:56:43 -0700 [thread overview]
Message-ID: <ZUPGu6jroYv3KFPv@google.com> (raw)
In-Reply-To: <64e3764e36ba7a00d94cc7db1dea1ef06b620aaf.camel@intel.com>
On Thu, Nov 02, 2023, Kai Huang wrote:
> On Wed, 2023-11-01 at 10:36 -0700, Sean Christopherson wrote:
> > On Wed, Nov 01, 2023, Kai Huang wrote:
> > >
> > > > +7.34 KVM_CAP_MEMORY_FAULT_INFO
> > > > +------------------------------
> > > > +
> > > > +:Architectures: x86
> > > > +:Returns: Informational only, -EINVAL on direct KVM_ENABLE_CAP.
> > > > +
> > > > +The presence of this capability indicates that KVM_RUN will fill
> > > > +kvm_run.memory_fault if KVM cannot resolve a guest page fault VM-Exit, e.g. if
> > > > +there is a valid memslot but no backing VMA for the corresponding host virtual
> > > > +address.
> > > > +
> > > > +The information in kvm_run.memory_fault is valid if and only if KVM_RUN returns
> > > > +an error with errno=EFAULT or errno=EHWPOISON *and* kvm_run.exit_reason is set
> > > > +to KVM_EXIT_MEMORY_FAULT.
> > >
> > > IIUC returning -EFAULT or whatever -errno is sort of KVM internal
> > > implementation.
> >
> > The errno that is returned to userspace is ABI. In KVM, it's a _very_ poorly
> > defined ABI for the vast majority of ioctls(), but it's still technically ABI.
> > KVM gets away with being cavalier with errno because the vast majority of errors
> > are considered fatal by userespace, i.e. in most cases, userspace simply doesn't
> > care about the exact errno.
> >
> > A good example is KVM_RUN with -EINTR; if KVM were to return something other than
> > -EINTR on a pending signal or vcpu->run->immediate_exit, userspace would fall over.
> >
> > > Is it better to relax the validity of kvm_run.memory_fault when
> > > KVM_RUN returns any -errno?
> >
> > Not unless there's a need to do so, and if there is then we can update the
> > documentation accordingly. If KVM's ABI is that kvm_run.memory_fault is valid
> > for any errno, then KVM would need to purge kvm_run.exit_reason super early in
> > KVM_RUN, e.g. to prevent an -EINTR return due to immediate_exit from being
> > misinterpreted as KVM_EXIT_MEMORY_FAULT. And purging exit_reason super early is
> > subtly tricky because KVM's (again, poorly documented) ABI is that *some* exit
> > reasons are preserved across KVM_RUN with vcpu->run->immediate_exit (or with a
> > pending signal).
> >
> > https://lore.kernel.org/all/ZFFbwOXZ5uI%2Fgdaf@google.com
> >
> >
>
> Agreed with not to relax to any errno. However using -EFAULT as part of ABI
> definition seems a little bit dangerous, e.g., someone could accidentally or
> mistakenly return -EFAULT in KVM_RUN at early time and/or in a completely
> different code path, etc. -EINTR has well defined meaning, but -EFAULT (which
> is "Bad address") seems doesn't but I am not sure either. :-)
KVM has returned -EFAULT since forever, i.e. it's effectively already part of the
ABI. I doubt there's a userspace that relies precisely on -EFAULT, but userspace
definitely will be confused if KVM returns '0' where KVM used to return -EFAULT.
And so if we want to return '0', it needs to be opt-in, which means forcing
userspace to enable a capability *and* requires code in KVM to conditionally return
'0' instead of -EFAULT/-EHWPOISON.
> One example is, for backing VMA with VM_IO | VM_PFNMAP, hva_to_pfn() returns
> KVM_PFN_ERR_FAULT when the kernel cannot get a valid PFN (e.g. when SGX vepc
> fault handler failed to allocate EPC) and kvm_handle_error_pfn() will just
> return -EFAULT. If kvm_run.exit_reason isn't purged early then is it possible
> to have some issue here?
Well, yeah, but that's exactly why this series has a patch to reset exit_reason.
The solution to "if KVM is buggy then bad things happen" is to not have KVM bugs :-)
next prev parent reply other threads:[~2023-11-02 15:56 UTC|newest]
Thread overview: 148+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-27 18:21 [PATCH v13 00/35] KVM: guest_memfd() and per-page attributes Sean Christopherson
2023-10-27 18:21 ` [PATCH v13 01/35] KVM: Tweak kvm_hva_range and hva_handler_t to allow reusing for gfn ranges Sean Christopherson
2023-11-01 12:46 ` Fuad Tabba
2023-10-27 18:21 ` [PATCH v13 02/35] KVM: Assert that mmu_invalidate_in_progress *never* goes negative Sean Christopherson
2023-10-30 16:27 ` Paolo Bonzini
2023-11-01 12:46 ` Fuad Tabba
2023-10-27 18:21 ` [PATCH v13 03/35] KVM: Use gfn instead of hva for mmu_notifier_retry Sean Christopherson
2023-10-30 16:30 ` Paolo Bonzini
2023-10-30 16:53 ` David Matlack
2023-10-30 17:00 ` Paolo Bonzini
2023-10-30 18:21 ` David Matlack
2023-10-30 18:19 ` David Matlack
2023-11-01 15:31 ` Xu Yilun
2023-10-27 18:21 ` [PATCH v13 04/35] KVM: WARN if there are dangling MMU invalidations at VM destruction Sean Christopherson
2023-10-30 16:32 ` Paolo Bonzini
2023-11-01 12:50 ` Fuad Tabba
2023-10-27 18:21 ` [PATCH v13 05/35] KVM: PPC: Drop dead code related to KVM_ARCH_WANT_MMU_NOTIFIER Sean Christopherson
2023-10-30 16:34 ` Paolo Bonzini
2023-11-01 12:51 ` Fuad Tabba
2023-10-27 18:21 ` [PATCH v13 06/35] KVM: PPC: Return '1' unconditionally for KVM_CAP_SYNC_MMU Sean Christopherson
2023-10-27 18:21 ` [PATCH v13 07/35] KVM: Convert KVM_ARCH_WANT_MMU_NOTIFIER to CONFIG_KVM_GENERIC_MMU_NOTIFIER Sean Christopherson
2023-10-30 16:37 ` Paolo Bonzini
2023-11-01 12:54 ` Fuad Tabba
2023-10-27 18:21 ` [PATCH v13 08/35] KVM: Introduce KVM_SET_USER_MEMORY_REGION2 Sean Christopherson
2023-10-30 16:41 ` Paolo Bonzini
2023-10-30 20:25 ` Sean Christopherson
2023-10-30 22:12 ` Sean Christopherson
2023-10-30 23:22 ` Paolo Bonzini
2023-10-31 0:18 ` Sean Christopherson
2023-10-31 2:26 ` Xiaoyao Li
2023-10-31 14:04 ` Sean Christopherson
2023-11-01 14:19 ` Fuad Tabba
2023-10-27 18:21 ` [PATCH v13 09/35] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report faults to userspace Sean Christopherson
2023-10-30 17:22 ` Paolo Bonzini
2023-11-01 7:30 ` Binbin Wu
2023-11-01 10:52 ` Huang, Kai
2023-11-01 17:36 ` Sean Christopherson
2023-11-02 2:19 ` Xiaoyao Li
2023-11-02 15:51 ` Sean Christopherson
2023-11-02 3:17 ` Huang, Kai
2023-11-02 9:35 ` Huang, Kai
2023-11-02 11:03 ` Paolo Bonzini
2023-11-02 15:44 ` Sean Christopherson
2023-11-02 18:35 ` Huang, Kai
2023-11-02 15:56 ` Sean Christopherson [this message]
2023-11-02 11:01 ` Paolo Bonzini
2023-11-03 4:09 ` Xu Yilun
2023-10-27 18:21 ` [PATCH v13 10/35] KVM: Add a dedicated mmu_notifier flag for reclaiming freed memory Sean Christopherson
2023-10-30 17:11 ` Paolo Bonzini
2023-11-02 13:55 ` Fuad Tabba
2023-10-27 18:21 ` [PATCH v13 11/35] KVM: Drop .on_unlock() mmu_notifier hook Sean Christopherson
2023-10-30 17:18 ` Paolo Bonzini
2023-11-02 13:55 ` Fuad Tabba
2023-10-27 18:21 ` [PATCH v13 12/35] KVM: Prepare for handling only shared mappings in mmu_notifier events Sean Christopherson
2023-10-30 17:21 ` Paolo Bonzini
2023-10-30 22:07 ` Sean Christopherson
2023-11-02 5:59 ` Binbin Wu
2023-11-02 11:14 ` Paolo Bonzini
2023-11-02 14:01 ` Fuad Tabba
2023-11-02 14:41 ` Sean Christopherson
2023-11-02 14:57 ` Fuad Tabba
2023-10-27 18:21 ` [PATCH v13 13/35] KVM: Introduce per-page memory attributes Sean Christopherson
2023-10-30 8:11 ` Chao Gao
2023-10-30 16:10 ` Sean Christopherson
2023-10-30 22:05 ` Sean Christopherson
2023-10-31 16:43 ` David Matlack
2023-11-02 3:01 ` Huang, Kai
2023-11-02 10:32 ` Paolo Bonzini
2023-11-02 10:55 ` Huang, Kai
2023-10-27 18:21 ` [PATCH v13 14/35] mm: Add AS_UNMOVABLE to mark mapping as completely unmovable Sean Christopherson
2023-10-30 17:24 ` Paolo Bonzini
2023-10-27 18:21 ` [PATCH v13 15/35] fs: Export anon_inode_getfile_secure() for use by KVM Sean Christopherson
2023-10-30 17:30 ` Paolo Bonzini
2023-11-02 16:24 ` Christian Brauner
2023-11-03 10:40 ` Paolo Bonzini
2023-10-27 18:21 ` [PATCH v13 16/35] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory Sean Christopherson
2023-10-31 2:27 ` Xiaoyao Li
2023-10-31 6:30 ` Chao Gao
2023-10-31 14:10 ` Sean Christopherson
2023-10-31 15:05 ` Fuad Tabba
2023-10-31 22:13 ` Sean Christopherson
2023-10-31 22:18 ` Paolo Bonzini
2023-11-01 10:51 ` Fuad Tabba
2023-11-01 21:55 ` Sean Christopherson
2023-11-02 13:52 ` Fuad Tabba
2023-11-03 23:17 ` Sean Christopherson
2023-10-31 18:24 ` David Matlack
2023-10-31 21:36 ` Sean Christopherson
2023-10-31 22:39 ` David Matlack
2023-11-02 15:48 ` Paolo Bonzini
2023-11-02 16:03 ` Sean Christopherson
2023-11-02 16:28 ` David Matlack
2023-11-02 17:37 ` Sean Christopherson
2023-11-03 9:42 ` Fuad Tabba
2023-11-04 10:26 ` Xu Yilun
2023-11-06 15:43 ` Sean Christopherson
2023-10-27 18:21 ` [PATCH v13 17/35] KVM: Add transparent hugepage support for dedicated guest memory Sean Christopherson
2023-10-31 8:35 ` Xiaoyao Li
2023-10-31 14:16 ` Sean Christopherson
2023-11-01 7:25 ` Xiaoyao Li
2023-11-01 13:41 ` Sean Christopherson
2023-11-01 13:49 ` Paolo Bonzini
2023-11-01 16:36 ` Sean Christopherson
2023-11-01 22:28 ` Paolo Bonzini
2023-11-01 22:34 ` Sean Christopherson
2023-11-01 23:17 ` Paolo Bonzini
2023-11-02 15:38 ` Sean Christopherson
2023-11-02 15:46 ` Paolo Bonzini
2023-11-27 11:13 ` Vlastimil Babka
2023-11-29 22:40 ` Sean Christopherson
2023-10-27 18:22 ` [PATCH v13 18/35] KVM: x86: "Reset" vcpu->run->exit_reason early in KVM_RUN Sean Christopherson
2023-10-30 17:31 ` Paolo Bonzini
2023-11-02 14:16 ` Fuad Tabba
2023-10-27 18:22 ` [PATCH v13 19/35] KVM: x86: Disallow hugepages when memory attributes are mixed Sean Christopherson
2023-10-27 18:22 ` [PATCH v13 20/35] KVM: x86/mmu: Handle page fault for private memory Sean Christopherson
2023-11-02 14:34 ` Fuad Tabba
2023-11-05 13:02 ` Xu Yilun
2023-11-05 16:19 ` Paolo Bonzini
2023-11-06 13:29 ` Xu Yilun
2023-11-06 15:56 ` Sean Christopherson
2023-10-27 18:22 ` [PATCH v13 21/35] KVM: Drop superfluous __KVM_VCPU_MULTIPLE_ADDRESS_SPACE macro Sean Christopherson
2023-11-02 14:35 ` Fuad Tabba
2023-10-27 18:22 ` [PATCH v13 22/35] KVM: Allow arch code to track number of memslot address spaces per VM Sean Christopherson
2023-10-30 17:34 ` Paolo Bonzini
2023-11-02 14:52 ` Fuad Tabba
2023-10-27 18:22 ` [PATCH v13 23/35] KVM: x86: Add support for "protected VMs" that can utilize private memory Sean Christopherson
2023-10-30 17:36 ` Paolo Bonzini
2023-11-06 11:00 ` Fuad Tabba
2023-11-06 11:03 ` Paolo Bonzini
2023-10-27 18:22 ` [PATCH v13 24/35] KVM: selftests: Drop unused kvm_userspace_memory_region_find() helper Sean Christopherson
2023-10-27 18:22 ` [PATCH v13 25/35] KVM: selftests: Convert lib's mem regions to KVM_SET_USER_MEMORY_REGION2 Sean Christopherson
2024-04-25 14:12 ` Dan Carpenter
2024-04-25 14:45 ` Shuah Khan
2024-04-25 15:09 ` Sean Christopherson
2024-04-25 16:22 ` Shuah Khan
2024-04-26 7:33 ` Jarkko Sakkinen
2023-10-27 18:22 ` [PATCH v13 26/35] KVM: selftests: Add support for creating private memslots Sean Christopherson
2023-10-27 18:22 ` [PATCH v13 27/35] KVM: selftests: Add helpers to convert guest memory b/w private and shared Sean Christopherson
2023-11-06 11:26 ` Fuad Tabba
2023-10-27 18:22 ` [PATCH v13 28/35] KVM: selftests: Add helpers to do KVM_HC_MAP_GPA_RANGE hypercalls (x86) Sean Christopherson
2023-10-27 18:22 ` [PATCH v13 29/35] KVM: selftests: Introduce VM "shape" to allow tests to specify the VM type Sean Christopherson
2023-10-27 18:22 ` [PATCH v13 30/35] KVM: selftests: Add GUEST_SYNC[1-6] macros for synchronizing more data Sean Christopherson
2023-10-27 18:22 ` [PATCH v13 31/35] KVM: selftests: Add x86-only selftest for private memory conversions Sean Christopherson
2023-10-27 18:22 ` [PATCH v13 32/35] KVM: selftests: Add KVM_SET_USER_MEMORY_REGION2 helper Sean Christopherson
2023-10-27 18:22 ` [PATCH v13 33/35] KVM: selftests: Expand set_memory_region_test to validate guest_memfd() Sean Christopherson
2023-10-27 18:22 ` [PATCH v13 34/35] KVM: selftests: Add basic selftest for guest_memfd() Sean Christopherson
2023-10-27 18:22 ` [PATCH v13 35/35] KVM: selftests: Test KVM exit behavior for private memory/access Sean Christopherson
2023-10-30 17:39 ` [PATCH v13 00/35] KVM: guest_memfd() and per-page attributes Paolo Bonzini
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