Date:   Thu, 2 Nov 2023 03:17:12 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "Christopherson,, Sean" <seanjc@...gle.com>
CC:     "Li, Xiaoyao" <xiaoyao.li@...el.com>,
        "kvm-riscv@...ts.infradead.org" <kvm-riscv@...ts.infradead.org>,
        "mic@...ikod.net" <mic@...ikod.net>,
        "liam.merwick@...cle.com" <liam.merwick@...cle.com>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "david@...hat.com" <david@...hat.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "amoorthy@...gle.com" <amoorthy@...gle.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "tabba@...gle.com" <tabba@...gle.com>,
        "kvmarm@...ts.linux.dev" <kvmarm@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "michael.roth@....com" <michael.roth@....com>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "oliver.upton@...ux.dev" <oliver.upton@...ux.dev>,
        "chao.p.peng@...ux.intel.com" <chao.p.peng@...ux.intel.com>,
        "palmer@...belt.com" <palmer@...belt.com>,
        "chenhuacai@...nel.org" <chenhuacai@...nel.org>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "Annapurve, Vishal" <vannapurve@...gle.com>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "mail@...iej.szmigiero.name" <mail@...iej.szmigiero.name>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "maz@...nel.org" <maz@...nel.org>,
        "willy@...radead.org" <willy@...radead.org>,
        "dmatlack@...gle.com" <dmatlack@...gle.com>,
        "anup@...infault.org" <anup@...infault.org>,
        "yu.c.zhang@...ux.intel.com" <yu.c.zhang@...ux.intel.com>,
        "Xu, Yilun" <yilun.xu@...el.com>,
        "qperret@...gle.com" <qperret@...gle.com>,
        "brauner@...nel.org" <brauner@...nel.org>,
        "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
        "ackerleytng@...gle.com" <ackerleytng@...gle.com>,
        "jarkko@...nel.org" <jarkko@...nel.org>,
        "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Wang, Wei W" <wei.w.wang@...el.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH v13 09/35] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report
 faults to userspace

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. :-)

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?

Powered by blists - more mailing lists