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Linux Fixing A "Hilarious/Revolting Performance Regression" Around Intel KVM Virtualization
It's not too often that 'fixes' to the Kernel-based Virtual Machine (KVM) are noteworthy but today is an interesting exception with among the KVM fixes sent in today ahead of the Linux 6.13-rc3 tagging is for beginning to deal with a 'hilarious/revolting' performance regression affecting recent generations of Intel processors
He sums up in the patch cover letter:"Fix a hilarious/revolting performance regression (relative to older CPU generations) in xstate_required_size() that pops up due to CPUID _in the host_ taking 3x-4x longer on Emerald Rapids than Skylake. That going to the mainline kernel today further sums up the situation and the higher Intel costs beginning with Emerald Rapids:"On Intel's Emerald Rapids, CPUID is *wildly* expensive, to the point where recomputing XSAVE offsets and sizes results in a 4x increase in latency of nested VM-Enter and VM-Exit (nested transitions can trigger xstate_required_size() multiple times per transition), relative to using cached values. With Intel Xeon Emerald Rapids having launched one year ago, it's surprising that it's taken until now to work on a solution to address these higher costs for the widely-used KVM that's an important piece of the Linux open-source virtualization stack.
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