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

From: [email-masked] (Christoffer Dall)
Date: Tue, 17 Oct 2017 07:34:35 -0700

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 <[email-masked]>
> >> ---
> >>  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 <[email-masked]>