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.
