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AMD CPUs currently execute WBINVD in the host when unregistering SEV guest memory or when deactivating SEV guests. Such cache maintenance is performed to prevent data corruption, wherein the encrypted (C=1) version of a dirty cache line might otherwise only be written back after the memory is written in a different context (ex: C=0), yielding corruption. However, WBINVD is performance-costly, especially because it invalidates processor caches. Strictly-speaking, unless the SEV ASID is being recycled (meaning the SNP firmware requires the use of WBINVD prior to DF_FLUSH), the cache invalidation triggered by WBINVD is unnecessary; only the writeback is needed to prevent data corruption in remaining scenarios. To improve performance in these scenarios, use WBNOINVD when available instead of WBINVD. WBNOINVD still writes back all dirty lines (preventing host data corruption by SEV guests) but does *not* invalidate processor caches. Note that the implementation of wbnoinvd() ensures fall back to WBINVD if WBNOINVD is unavailable. In anticipation of forthcoming optimizations to limit the WBNOINVD only to physical CPUs that have executed SEV guests, place the call to wbnoinvd_on_all_cpus() in a wrapper function sev_writeback_caches(). Signed-off-by: Kevin Loughlin <kevinloughlin@google.com> Reviewed-by: Mingwei Zhang <mizhang@google.com> Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com> Link: https://lore.kernel.org/r/20250201000259.3289143-3-kevinloughlin@google.com [sean: tweak comment regarding CLFUSH] Cc: Francesco Lavra <francescolavra.fl@gmail.com> Link: https://lore.kernel.org/r/20250522233733.3176144-8-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 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.