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6783ca4105
Preserve or clobber all GPRs (except RIP and RSP, as they're saved and restored via the VMCS) when performing a ucall on x86 to fudge around a horrific long-standing bug in selftests' nested VMX support where L2's GPRs are not preserved across a nested VM-Exit. I.e. if a test triggers a nested VM-Exit to L1 in response to a ucall, e.g. GUEST_SYNC(), then L2's GPR state can be corrupted. The issues manifests as an unexpected #GP in clear_bit() when running the hyperv_evmcs test due to RBX being used to track the ucall object, and RBX being clobbered by the nested VM-Exit. The problematic hyperv_evmcs testcase is where L0 (test's host userspace) injects an NMI in response to GUEST_SYNC(8) from L2, but the bug could "randomly" manifest in any test that induces a nested VM-Exit from L0. The bug hasn't caused failures in the past due to sheer dumb luck. The obvious fix is to rework the nVMX helpers to save/restore L2 GPRs across VM-Exit and VM-Enter, but that is a much bigger task and carries its own risks, e.g. nSVM does save/restore GPRs, but not in a thread-safe manner, and there is a _lot_ of cleanup that can be done to unify code for doing VM-Enter on nVMX, nSVM, and eVMCS. Link: https://lore.kernel.org/r/20230729003643.1053367-4-seanjc@google.com Signed-off-by: Sean Christopherson <seanjc@google.com>
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.