On Sun, May 28, 2017 at 10:00:17AM -0700, Andy Lutomirski wrote: > When PCID is enabled, CR3's PCID bits can change during context > switches, so KVM won't be able to treat CR3 as a per-mm constant any > more. > > I structured this like the existing CR4 handling. Under ordinary > circumstances (PCID disabled or if the current PCID and the value > that's already in the VMCS match), then we won't do an extra VMCS > write, and we'll never do an extra direct CR3 read. The overhead > should be minimal. > > I disallowed using the new helper in non-atomic context because > PCID support will cause CR3 to stop being constant in non-atomic > process context. > > (Frankly, it also scares me a bit that KVM ever treated CR3 as > constant, but it looks like it was okay before.) > > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> > Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx> > Cc: kvm@xxxxxxxxxxxxxxx > Cc: Rik van Riel <riel@xxxxxxxxxx> > Cc: Dave Hansen <dave.hansen@xxxxxxxxx> > Cc: Nadav Amit <namit@xxxxxxxxxx> > Cc: Michal Hocko <mhocko@xxxxxxxx> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx> > Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx> > --- > arch/x86/include/asm/mmu_context.h | 19 +++++++++++++++++++ > arch/x86/kvm/vmx.c | 21 ++++++++++++++++++--- > 2 files changed, 37 insertions(+), 3 deletions(-) > > diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h > index 187c39470a0b..f20d7ea47095 100644 > --- a/arch/x86/include/asm/mmu_context.h > +++ b/arch/x86/include/asm/mmu_context.h > @@ -266,4 +266,23 @@ static inline bool arch_vma_access_permitted(struct vm_area_struct *vma, > return __pkru_allows_pkey(vma_pkey(vma), write); > } > > + > +/* > + * This can be used from process context to figure out what the value of > + * CR3 is without needing to do a (slow) read_cr3(). > + * > + * It's intended to be used for code like KVM that sneakily changes CR3 > + * and needs to restore it. It needs to be used very carefully. > + */ > +static inline unsigned long __get_current_cr3_fast(void) > +{ > + unsigned long cr3 = __pa(this_cpu_read(cpu_tlbstate.loaded_mm)->pgd); > + > + /* For now, be very restrictive about when this can be called. */ > + VM_WARN_ON(in_nmi() || !in_atomic()); With the following config (from Fedora26 + olddefconfig) $ grep PREEMPT .config CONFIG_PREEMPT_NOTIFIERS=y # CONFIG_PREEMPT_NONE is not set CONFIG_PREEMPT_VOLUNTARY=y # CONFIG_PREEMPT is not set I hit this warning on !in_atomic() on every vm entry. Shouldn't this be preemptible() instead? Roman. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>