5.15-stable review patch. If anyone has any objections, please let me know.
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From: Sean Christopherson seanjc@google.com
This reverts commit 71ba3f3189c78f756a659568fb473600fd78f207.
Disable the TDP MMU by default in v5.15 kernels to "fix" several severe performance bugs that have since been found and fixed in the TDP MMU, but are unsuitable for backporting to v5.15.
The problematic bugs are fixed by upstream commit edbdb43fc96b ("KVM: x86: Preserve TDP MMU roots until they are explicitly invalidated") and commit 01b31714bd90 ("KVM: x86: Do not unload MMU roots when only toggling CR0.WP with TDP enabled"). Both commits fix scenarios where KVM will rebuild all TDP MMU page tables in paths that are frequently hit by certain guest workloads. While not exactly common, the guest workloads are far from rare. The fallout of rebuilding TDP MMU page tables can be so severe in some cases that it induces soft lockups in the guest.
Commit edbdb43fc96b would require _significant_ effort and churn to backport due it depending on a major rework that was done in v5.18.
Commit 01b31714bd90 has far fewer direct conflicts, but has several subtle _known_ dependencies, and it's unclear whether or not there are more unknown dependencies that have been missed.
Lastly, disabling the TDP MMU in v5.15 kernels also fixes a lurking train wreck started by upstream commit a955cad84cda ("KVM: x86/mmu: Retry page fault if root is invalidated by memslot update"). That commit was tagged for stable to fix a memory leak, but didn't cherry-pick cleanly and was never backported to v5.15. Which is extremely fortunate, as it introduced not one but two bugs, one of which was fixed by upstream commit 18c841e1f411 ("KVM: x86: Retry page fault if MMU reload is pending and root has no sp"), while the other was unknowingly fixed by upstream commit ba6e3fe25543 ("KVM: x86/mmu: Grab mmu_invalidate_seq in kvm_faultin_pfn()") in v6.3 (a one-off fix will be made for v6.1 kernels, which did receive a backport for a955cad84cda). Disabling the TDP MMU by default reduces the probability of breaking v5.15 kernels by backporting only a subset of the fixes.
As far as what is lost by disabling the TDP MMU, the main selling point of the TDP MMU is its ability to service page fault VM-Exits in parallel, i.e. the main benefactors of the TDP MMU are deployments of large VMs (hundreds of vCPUs), and in particular delployments that live-migrate such VMs and thus need to fault-in huge amounts of memory on many vCPUs after restarting the VM after migration.
Smaller VMs can see performance improvements, but nowhere enough to make up for the TDP MMU (in v5.15) absolutely cratering performance for some workloads. And practically speaking, anyone that is deploying and migrating VMs with hundreds of vCPUs is likely rolling their own kernel, not using a stock v5.15 series kernel.
Link: https://lore.kernel.org/all/ZDmEGM+CgYpvDLh6@google.com Link: https://lore.kernel.org/all/f023d927-52aa-7e08-2ee5-59a2fbc65953@gameservers... Acked-by: Mathias Krause minipli@grsecurity.net Acked-by: Jeremi Piotrowski jpiotrowski@linux.microsoft.com Signed-off-by: Sean Christopherson seanjc@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/kvm/mmu/tdp_mmu.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/arch/x86/kvm/mmu/tdp_mmu.c +++ b/arch/x86/kvm/mmu/tdp_mmu.c @@ -10,7 +10,7 @@ #include <asm/cmpxchg.h> #include <trace/events/kvm.h>
-static bool __read_mostly tdp_mmu_enabled = true; +static bool __read_mostly tdp_mmu_enabled = false; module_param_named(tdp_mmu, tdp_mmu_enabled, bool, 0644);
/* Initializes the TDP MMU for the VM, if enabled. */