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0e3223d8d0
When attempting to allocate a shadow root for a !visible guest root gfn, e.g. that resides in MMIO space, load a dummy root that is backed by the zero page instead of immediately synthesizing a triple fault shutdown (using the zero page ensures any attempt to translate memory will generate a !PRESENT fault and thus VM-Exit). Unless the vCPU is racing with memslot activity, KVM will inject a page fault due to not finding a visible slot in FNAME(walk_addr_generic), i.e. the end result is mostly same, but critically KVM will inject a fault only *after* KVM runs the vCPU with the bogus root. Waiting to inject a fault until after running the vCPU fixes a bug where KVM would bail from nested VM-Enter if L1 tried to run L2 with TDP enabled and a !visible root. Even though a bad root will *probably* lead to shutdown, (a) it's not guaranteed and (b) the CPU won't read the underlying memory until after VM-Enter succeeds. E.g. if L1 runs L2 with a VMX preemption timer value of '0', then architecturally the preemption timer VM-Exit is guaranteed to occur before the CPU executes any instruction, i.e. before the CPU needs to translate a GPA to a HPA (so long as there are no injected events with higher priority than the preemption timer). If KVM manages to get to FNAME(fetch) with a dummy root, e.g. because userspace created a memslot between installing the dummy root and handling the page fault, simply unload the MMU to allocate a new root and retry the instruction. Use KVM_REQ_MMU_FREE_OBSOLETE_ROOTS to drop the root, as invoking kvm_mmu_free_roots() while holding mmu_lock would deadlock, and conceptually the dummy root has indeeed become obsolete. The only difference versus existing usage of KVM_REQ_MMU_FREE_OBSOLETE_ROOTS is that the root has become obsolete due to memslot *creation*, not memslot deletion or movement. Reported-by: Reima Ishii <ishiir@g.ecc.u-tokyo.ac.jp> Cc: Yu Zhang <yu.c.zhang@linux.intel.com> Link: https://lore.kernel.org/r/20230729005200.1057358-6-seanjc@google.com Signed-off-by: Sean Christopherson <seanjc@google.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.