Re: [PATCH v4 08/19] arm64: KVM: Dynamically patch the kernel/hyp VA mask

[Christoffer Dall <christoffer.dall@linaro.org>]

On Thu, Feb 15, 2018 at 01:11:02PM +0000, Marc Zyngier wrote:
> On 15/01/18 11:47, Christoffer Dall wrote:
> > On Thu, Jan 04, 2018 at 06:43:23PM +0000, Marc Zyngier wrote:
> >> So far, we're using a complicated sequence of alternatives to
> >> patch the kernel/hyp VA mask on non-VHE, and NOP out the
> >> masking altogether when on VHE.
> >>
> >> THe newly introduced dynamic patching gives us the opportunity
> > 
> > nit: s/THe/The/
> > 
> >> to simplify that code by patching a single instruction with
> >> the correct mask (instead of the mind bending cummulative masking
> >> we have at the moment) or even a single NOP on VHE.
> >>
> >> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> >> ---
> >>  arch/arm64/include/asm/kvm_mmu.h | 45 ++++++--------------
> >>  arch/arm64/kvm/Makefile          |  2 +-
> >>  arch/arm64/kvm/va_layout.c       | 91 ++++++++++++++++++++++++++++++++++++++++
> >>  3 files changed, 104 insertions(+), 34 deletions(-)
> >>  create mode 100644 arch/arm64/kvm/va_layout.c
> >>
> >> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> >> index 672c8684d5c2..b0c3cbe9b513 100644
> >> --- a/arch/arm64/include/asm/kvm_mmu.h
> >> +++ b/arch/arm64/include/asm/kvm_mmu.h
> >> @@ -69,9 +69,6 @@
> >>   * mappings, and none of this applies in that case.
> >>   */
> >>  
> >> -#define HYP_PAGE_OFFSET_HIGH_MASK	((UL(1) << VA_BITS) - 1)
> >> -#define HYP_PAGE_OFFSET_LOW_MASK	((UL(1) << (VA_BITS - 1)) - 1)
> >> -
> >>  #ifdef __ASSEMBLY__
> >>  
> >>  #include <asm/alternative.h>
> >> @@ -81,28 +78,14 @@
> >>   * Convert a kernel VA into a HYP VA.
> >>   * reg: VA to be converted.
> >>   *
> >> - * This generates the following sequences:
> >> - * - High mask:
> >> - *		and x0, x0, #HYP_PAGE_OFFSET_HIGH_MASK
> >> - *		nop
> >> - * - Low mask:
> >> - *		and x0, x0, #HYP_PAGE_OFFSET_HIGH_MASK
> >> - *		and x0, x0, #HYP_PAGE_OFFSET_LOW_MASK
> >> - * - VHE:
> >> - *		nop
> >> - *		nop
> >> - *
> >> - * The "low mask" version works because the mask is a strict subset of
> >> - * the "high mask", hence performing the first mask for nothing.
> >> - * Should be completely invisible on any viable CPU.
> >> + * The actual code generation takes place in kvm_update_va_mask, and
> >> + * the instructions below are only there to reserve the space and
> >> + * perform the register allocation.
> > 
> > Not just register allocation, but also to tell the generating code which
> > registers to use for its operands, right?
> 
> That's what I meant by register allocation.
> 

I suppose that's included in the term.  My confusion was that I
initially looked at this like the clobber list in inline asm, but then
realized that you really use the specific registers for each instruction
in the order listed here.

> > 
> >>   */
> >>  .macro kern_hyp_va	reg
> >> -alternative_if_not ARM64_HAS_VIRT_HOST_EXTN
> >> -	and     \reg, \reg, #HYP_PAGE_OFFSET_HIGH_MASK
> >> -alternative_else_nop_endif
> >> -alternative_if ARM64_HYP_OFFSET_LOW
> >> -	and     \reg, \reg, #HYP_PAGE_OFFSET_LOW_MASK
> >> -alternative_else_nop_endif
> >> +alternative_cb kvm_update_va_mask
> >> +	and     \reg, \reg, #1
> >> +alternative_cb_end
> >>  .endm
> >>  
> >>  #else
> >> @@ -113,18 +96,14 @@ alternative_else_nop_endif
> >>  #include <asm/mmu_context.h>
> >>  #include <asm/pgtable.h>
> >>  
> >> +void kvm_update_va_mask(struct alt_instr *alt,
> >> +			__le32 *origptr, __le32 *updptr, int nr_inst);
> >> +
> >>  static inline unsigned long __kern_hyp_va(unsigned long v)
> >>  {
> >> -	asm volatile(ALTERNATIVE("and %0, %0, %1",
> >> -				 "nop",
> >> -				 ARM64_HAS_VIRT_HOST_EXTN)
> >> -		     : "+r" (v)
> >> -		     : "i" (HYP_PAGE_OFFSET_HIGH_MASK));
> >> -	asm volatile(ALTERNATIVE("nop",
> >> -				 "and %0, %0, %1",
> >> -				 ARM64_HYP_OFFSET_LOW)
> >> -		     : "+r" (v)
> >> -		     : "i" (HYP_PAGE_OFFSET_LOW_MASK));
> >> +	asm volatile(ALTERNATIVE_CB("and %0, %0, #1\n",
> >> +				    kvm_update_va_mask)
> >> +		     : "+r" (v));
> >>  	return v;
> >>  }
> >>  
> >> diff --git a/arch/arm64/kvm/Makefile b/arch/arm64/kvm/Makefile
> >> index 87c4f7ae24de..93afff91cb7c 100644
> >> --- a/arch/arm64/kvm/Makefile
> >> +++ b/arch/arm64/kvm/Makefile
> >> @@ -16,7 +16,7 @@ kvm-$(CONFIG_KVM_ARM_HOST) += $(KVM)/kvm_main.o $(KVM)/coalesced_mmio.o $(KVM)/e
> >>  kvm-$(CONFIG_KVM_ARM_HOST) += $(KVM)/arm/arm.o $(KVM)/arm/mmu.o $(KVM)/arm/mmio.o
> >>  kvm-$(CONFIG_KVM_ARM_HOST) += $(KVM)/arm/psci.o $(KVM)/arm/perf.o
> >>  
> >> -kvm-$(CONFIG_KVM_ARM_HOST) += inject_fault.o regmap.o
> >> +kvm-$(CONFIG_KVM_ARM_HOST) += inject_fault.o regmap.o va_layout.o
> >>  kvm-$(CONFIG_KVM_ARM_HOST) += hyp.o hyp-init.o handle_exit.o
> >>  kvm-$(CONFIG_KVM_ARM_HOST) += guest.o debug.o reset.o sys_regs.o sys_regs_generic_v8.o
> >>  kvm-$(CONFIG_KVM_ARM_HOST) += vgic-sys-reg-v3.o
> >> diff --git a/arch/arm64/kvm/va_layout.c b/arch/arm64/kvm/va_layout.c
> >> new file mode 100644
> >> index 000000000000..aee758574e61
> >> --- /dev/null
> >> +++ b/arch/arm64/kvm/va_layout.c
> >> @@ -0,0 +1,91 @@
> >> +/*
> >> + * Copyright (C) 2017 ARM Ltd.
> >> + * Author: Marc Zyngier <marc.zyngier@arm.com>
> >> + *
> >> + * This program is free software; you can redistribute it and/or modify
> >> + * it under the terms of the GNU General Public License version 2 as
> >> + * published by the Free Software Foundation.
> >> + *
> >> + * This program is distributed in the hope that it will be useful,
> >> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> >> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> >> + * GNU General Public License for more details.
> >> + *
> >> + * You should have received a copy of the GNU General Public License
> >> + * along with this program.  If not, see <http://www.gnu.org/licenses/>.
> >> + */
> >> +
> >> +#include <linux/kvm_host.h>
> >> +#include <asm/alternative.h>
> >> +#include <asm/debug-monitors.h>
> >> +#include <asm/insn.h>
> >> +#include <asm/kvm_mmu.h>
> >> +
> >> +#define HYP_PAGE_OFFSET_HIGH_MASK	((UL(1) << VA_BITS) - 1)
> >> +#define HYP_PAGE_OFFSET_LOW_MASK	((UL(1) << (VA_BITS - 1)) - 1)
> >> +
> >> +static u64 va_mask;
> >> +
> >> +static void compute_layout(void)
> >> +{
> >> +	phys_addr_t idmap_addr = __pa_symbol(__hyp_idmap_text_start);
> >> +	unsigned long mask = HYP_PAGE_OFFSET_HIGH_MASK;
> >> +
> >> +	/*
> >> +	 * Activate the lower HYP offset only if the idmap doesn't
> >> +	 * clash with it,
> >> +	 */
> > 
> > The commentary is a bit strange given the logic below...
> > 
> >> +	if (idmap_addr > HYP_PAGE_OFFSET_LOW_MASK)
> >> +		mask = HYP_PAGE_OFFSET_HIGH_MASK;
> > 
> > ... was the initialization supposed to be LOW_MASK?
> > 
> > (and does this imply that this was never tested on a system that
> > actually used the low mask?)
> 
> I must has messed up with a later refactoring, as that code gets
> replaced pretty quickly in the series. The full series was definitely
> tested on Seattle with 39bit VAs, which is the only configuration I have
> that triggers the low mask.
> 

I realized later that this was just a temporary thing in the patch
series. Still, for posterity, we should probably fix it up.

> > 
> >> +
> >> +	va_mask = mask;
> >> +}
> >> +
> >> +static u32 compute_instruction(int n, u32 rd, u32 rn)
> >> +{
> >> +	u32 insn = AARCH64_BREAK_FAULT;
> >> +
> >> +	switch (n) {
> >> +	case 0:
> > 
> > hmmm, wonder why we need this n==0 check...
> 
> Makes patch splitting a bit easier. I can rework that if that helps.
> 

No need, I get it now.

> > 
> >> +		insn = aarch64_insn_gen_logical_immediate(AARCH64_INSN_LOGIC_AND,
> >> +							  AARCH64_INSN_VARIANT_64BIT,
> >> +							  rn, rd, va_mask);
> >> +		break;
> >> +	}
> >> +
> >> +	return insn;
> >> +}
> >> +
> >> +void __init kvm_update_va_mask(struct alt_instr *alt,
> >> +			       __le32 *origptr, __le32 *updptr, int nr_inst)
> >> +{
> >> +	int i;
> >> +
> >> +	/* We only expect a 1 instruction sequence */
> > 
> > nit: wording is a bit strange, how about
> > "We only expect a single instruction in the alternative sequence"
> 
> Sure.
> 
> > 
> >> +	BUG_ON(nr_inst != 1);
> >> +
> >> +	if (!has_vhe() && !va_mask)
> >> +		compute_layout();
> >> +
> >> +	for (i = 0; i < nr_inst; i++) {
> > 
> > It's a bit funny to have a loop with the above BUG_ON.
> > 
> > (I'm beginning to wonder if a future patch expands on this single
> > instruction thing, in which case a hint in the commit message would have
> > been nice.)
> 
> That's indeed what is happening. A further patch expands the single
> instruction to a 5 instruction sequence. I'll add a comment to that effect.
> 

Thanks,
-Christoffer