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KVM: x86: Use "checked" versions of get_user() and put_user()

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Use the normal, checked versions for get_user() and put_user() instead of
the double-underscore versions that omit range checks, as the checked
versions are actually measurably faster on modern CPUs (12%+ on Intel,
25%+ on AMD).

The performance hit on the unchecked versions is almost entirely due to
the added LFENCE on CPUs where LFENCE is serializing (which is effectively
all modern CPUs), which was added by commit 304ec1b050 ("x86/uaccess:
Use __uaccess_begin_nospec() and uaccess_try_nospec").  The small
optimizations done by commit b19b74bc99 ("x86/mm: Rework address range
check in get_user() and put_user()") likely shave a few cycles off, but
the bulk of the extra latency comes from the LFENCE.

Don't bother trying to open-code an equivalent for performance reasons, as
the loss of inlining (e.g. see commit ea6f043fc9 ("x86: Make __get_user()
generate an out-of-line call") is largely a non-factor (ignoring setups
where RET is something entirely different),

As measured across tens of millions of calls of guest PTE reads in
FNAME(walk_addr_generic):

              __get_user()  get_user()  open-coded  open-coded, no LFENCE
Intel (EMR)           75.1        67.6        75.3                   65.5
AMD (Turin)           68.1        51.1        67.5                   49.3

Note, Hyper-V MSR emulation is not a remotely hot path, but convert it
anyways for consistency, and because there is a general desire to remove
__{get,put}_user() entirely.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Closes: https://lore.kernel.org/all/CAHk-=wimh_3jM9Xe8Zx0rpuf8CPDu6DkRCGb44azk0Sz5yqSnw@mail.gmail.com
Cc: Borislav Petkov <bp@alien8.de>
Link: https://patch.msgid.link/20251106210206.221558-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson
2025-11-06 13:02:06 -08:00
parent 995d504100
commit b3e5b670c9
2 changed files with 2 additions and 2 deletions
Download Patch File Download Diff File Expand all files Collapse all files

2
arch/x86/kvm/hyperv.c
View File

@@ -1568,7 +1568,7 @@ static int kvm_hv_set_msr(struct kvm_vcpu *vcpu, u32 msr, u64 data, bool host)
* only, there can be valuable data in the rest which needs
* to be preserved e.g. on migration.
*/
if (__put_user(0, (u32 __user *)addr))
if (put_user(0, (u32 __user *)addr))
return 1;
hv_vcpu->hv_vapic = data;
kvm_vcpu_mark_page_dirty(vcpu, gfn);

2
arch/x86/kvm/mmu/paging_tmpl.h
View File

@@ -402,7 +402,7 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
goto error;
ptep_user = (pt_element_t __user *)((void *)host_addr + offset);
if (unlikely(__get_user(pte, ptep_user)))
if (unlikely(get_user(pte, ptep_user)))
goto error;
walker->ptep_user[walker->level - 1] = ptep_user;