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Sean Christopherson 3fe48378e5 KVM: TDX: Explicitly set user-return MSRs that *may* be clobbered by the TDX-Module 
commit c0711f8c61 upstream.

Set all user-return MSRs to their post-TD-exit value when preparing to run
a TDX vCPU to ensure the value that KVM expects to be loaded after running
the vCPU is indeed the value that's loaded in hardware.  If the TDX-Module
doesn't actually enter the guest, i.e. doesn't do VM-Enter, then it won't
"restore" VMM state, i.e. won't clobber user-return MSRs to their expected
post-run values, in which case simply updating KVM's "cached" value will
effectively corrupt the cache due to hardware still holding the original
value.

In theory, KVM could conditionally update the current user-return value if
and only if tdh_vp_enter() succeeds, but in practice "success" doesn't
guarantee the TDX-Module actually entered the guest, e.g. if the TDX-Module
synthesizes an EPT Violation because it suspects a zero-step attack.

Force-load the expected values instead of trying to decipher whether or
not the TDX-Module restored/clobbered MSRs, as the risk doesn't justify
the benefits.  Effectively avoiding four WRMSRs once per run loop (even if
the vCPU is scheduled out, user-return MSRs only need to be reloaded if
the CPU exits to userspace or runs a non-TDX vCPU) is likely in the noise
when amortized over all entries, given the cost of running a TDX vCPU.
E.g. the cost of the WRMSRs is somewhere between ~300 and ~500 cycles,
whereas the cost of a _single_ roundtrip to/from a TDX guest is thousands
of cycles.

Fixes: e0b4f31a3c ("KVM: TDX: restore user ret MSRs")
Cc: stable@vger.kernel.org
Cc: Yan Zhao <yan.y.zhao@intel.com>
Cc: Xiaoyao Li <xiaoyao.li@intel.com>
Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://patch.msgid.link/20251030191528.3380553-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2026-01-02 12:57:21 +01:00
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README 

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 reStructuredText 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.