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authorLinus Torvalds <torvalds@linux-foundation.org>2020-08-23 21:21:16 +0300
committerLinus Torvalds <torvalds@linux-foundation.org>2020-08-23 21:21:16 +0300
commit550c2129d93d5eb198835ac83c05ef672e8c491c (patch)
tree922fe5453bb6d9c4e0c8091d2d3149089706f93d
parentcea05c192b07b82a770816fc9d06031403cea164 (diff)
parent6a3ea3e68b8a8a26c4aaac03432ed92269c9a14e (diff)
downloadlinux-550c2129d93d5eb198835ac83c05ef672e8c491c.tar.xz
Merge tag 'x86-urgent-2020-08-23' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull x86 fix from Thomas Gleixner: "A single fix for x86 which removes the RDPID usage from the paranoid entry path and unconditionally uses LSL to retrieve the CPU number. RDPID depends on MSR_TSX_AUX. KVM has an optmization to avoid expensive MRS read/writes on VMENTER/EXIT. It caches the MSR values and restores them either when leaving the run loop, on preemption or when going out to user space. MSR_TSX_AUX is part of that lazy MSR set, so after writing the guest value and before the lazy restore any exception using the paranoid entry will read the guest value and use it as CPU number to retrieve the GSBASE value for the current CPU when FSGSBASE is enabled. As RDPID is only used in that particular entry path, there is no reason to burden VMENTER/EXIT with two extra MSR writes. Remove the RDPID optimization, which is not even backed by numbers from the paranoid entry path instead" * tag 'x86-urgent-2020-08-23' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: x86/entry/64: Do not use RDPID in paranoid entry to accomodate KVM
-rw-r--r--arch/x86/entry/calling.h10
1 files changed, 6 insertions, 4 deletions
diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index 98e4d8886f11..ae9b0d4615b3 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -374,12 +374,14 @@ For 32-bit we have the following conventions - kernel is built with
* Fetch the per-CPU GSBASE value for this processor and put it in @reg.
* We normally use %gs for accessing per-CPU data, but we are setting up
* %gs here and obviously can not use %gs itself to access per-CPU data.
+ *
+ * Do not use RDPID, because KVM loads guest's TSC_AUX on vm-entry and
+ * may not restore the host's value until the CPU returns to userspace.
+ * Thus the kernel would consume a guest's TSC_AUX if an NMI arrives
+ * while running KVM's run loop.
*/
.macro GET_PERCPU_BASE reg:req
- ALTERNATIVE \
- "LOAD_CPU_AND_NODE_SEG_LIMIT \reg", \
- "RDPID \reg", \
- X86_FEATURE_RDPID
+ LOAD_CPU_AND_NODE_SEG_LIMIT \reg
andq $VDSO_CPUNODE_MASK, \reg
movq __per_cpu_offset(, \reg, 8), \reg
.endm