Re: [PATCH 03/10] arm: KVM: Add optimized PIPT icache flushing

Christoffer Dall
2017-10-17T14:34:35Z

On Tue, Oct 17, 2017 at 10:26:31AM +0100, Marc Zyngier wrote:
> On 16/10/17 21:07, Christoffer Dall wrote:
> > On Mon, Oct 09, 2017 at 04:20:25PM +0100, Marc Zyngier wrote:
> >> Calling __cpuc_coherent_user_range to invalidate the icache on
> >> a PIPT icache machine has some pointless overhead, as it starts
> >> by cleaning the dcache to the PoU, while we're guaranteed to
> >> have already cleaned it to the PoC.
> >>
> >> As KVM is the only user of such a feature, let's implement some
> >> ad-hoc cache flushing in kvm_mmu.h. Should it become useful to
> >> other subsystems, it can be moved to a more global location.
> >>
> >> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> >> ---
> >> arch/arm/include/asm/kvm_hyp.h | 2 ++
> >> arch/arm/include/asm/kvm_mmu.h | 24 ++++++++++++++++++++++--
> >> 2 files changed, 24 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/arch/arm/include/asm/kvm_hyp.h b/arch/arm/include/asm/kvm_hyp.h
> >> index 14b5903f0224..ad541f9ecc78 100644
> >> --- a/arch/arm/include/asm/kvm_hyp.h
> >> +++ b/arch/arm/include/asm/kvm_hyp.h
> >> @@ -69,6 +69,8 @@
> >> #define HIFAR __ACCESS_CP15(c6, 4, c0, 2)
> >> #define HPFAR __ACCESS_CP15(c6, 4, c0, 4)
> >> #define ICIALLUIS __ACCESS_CP15(c7, 0, c1, 0)
> >> +#define BPIALLIS __ACCESS_CP15(c7, 0, c1, 6)
> >> +#define ICIMVAU __ACCESS_CP15(c7, 0, c5, 1)
> >> #define ATS1CPR __ACCESS_CP15(c7, 0, c8, 0)
> >> #define TLBIALLIS __ACCESS_CP15(c8, 0, c3, 0)
> >> #define TLBIALL __ACCESS_CP15(c8, 0, c7, 0)
> >> diff --git a/arch/arm/include/asm/kvm_mmu.h b/arch/arm/include/asm/kvm_mmu.h
> >> index f553aa62d0c3..6773dcf21bff 100644
> >> --- a/arch/arm/include/asm/kvm_mmu.h
> >> +++ b/arch/arm/include/asm/kvm_mmu.h
> >> @@ -37,6 +37,8 @@
> >>
> >> #include <linux/highmem.h>
> >> #include <asm/cacheflush.h>
> >> +#include <asm/cputype.h>
> >> +#include <asm/kvm_hyp.h>
> >> #include <asm/pgalloc.h>
> >> #include <asm/stage2_pgtable.h>
> >>
> >> @@ -157,6 +159,8 @@ static inline void __coherent_icache_guest_page(struct kvm_vcpu *vcpu,
> >> kvm_pfn_t pfn,
> >> unsigned long size)
> >> {
> >> + u32 iclsz;
> >> +
> >> /*
> >> * If we are going to insert an instruction page and the icache is
> >> * either VIPT or PIPT, there is a potential problem where the host
> >> @@ -182,17 +186,33 @@ static inline void __coherent_icache_guest_page(struct kvm_vcpu *vcpu,
> >> }
> >>
> >> /* PIPT cache. As for the d-side, use a temporary kernel mapping. */
> >> + iclsz = 4 << (read_cpuid(CPUID_CACHETYPE) & 0xf);
> >> +
> >
> > nit: the 4 here is a bit cryptic, could we say something like (perhaps
> > slightly over-explained):
> > /*
> > * CTR IminLine contains Log2 of the number of words in the cache line,
> > * so we can get the number of words as 2 << (IminLine - 1). To get the
> > * number of bytes, we multiply by 4 (the number of bytes in a 32-bit
> > * word), and get 4 << (IminLine).
> > */
>
> Absolutely. I'll fold that in. Thanks.
>
> >> while (size) {
> >> void *va = kmap_atomic_pfn(pfn);
> >> + void *end = va + PAGE_SIZE;
> >> + void *addr = va;
> >> +
> >> + do {
> >> + write_sysreg(addr, ICIMVAU);
> >
> > Maybe an oddball place to ask this, but I don't recall why we need PoU
> > everywhere, would PoC potentially be enough?
>
> PoC is in general stronger than PoU. All we care is for instructions to
> be fetched from the point where the icache cannot be distinguished from
> the dcache - the definition of the PoU. Also, I don't think there is a
> way to invalidate the icache to the PoC.
>

Doi, I switched the meaning around in my head. Sorry about the noise.

> >
> >> + addr += iclsz;
> >> + } while (addr < end);
> >>
> >> - __cpuc_coherent_user_range((unsigned long)va,
> >> - (unsigned long)va + PAGE_SIZE);
> >> + dsb(ishst);
> >> + isb();
> >
> > Do we really need this in every iteration of the loop?
>
> The problem is that we need to factor in the interaction with the unmap
> below. If we don't enforce the invalidation now, we may unmap the page
> before the invalidations are finished, with could lead to a page fault.
> If we didn't have to deal with highmem, that would indeed be a good
> optimization.
>

Ah, I completely failed to see that. Thanks for the explanation.

> >
> >>
> >> size -= PAGE_SIZE;
> >> pfn++;
> >>
> >> kunmap_atomic(va);
> >> }
> >> +
> >> + /* Check if we need to invalidate the BTB */
> >> + if ((read_cpuid_ext(CPUID_EXT_MMFR1) >> 24) != 4) {
> >
> > Either I'm having a bad day or you meant to shift this 28, not 24?
>
> Oops... That's indeed totally broken. Thanks for noticing it!
>
With that fixed:

Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>