Subject: [PATCH] arm64: kvm: Fix kvm teardown for systems using the extended idmap

From: [email-masked] (Marc Zyngier)
Date: Mon, 2 May 2016 13:05:35 +0100

On Fri, 29 Apr 2016 18:27:03 +0100
James Morse <[email-masked]> wrote:

Hi James,

> If memory is located above 1<<VA_BITS, kvm adds an extra level to its page
> tables, merging the runtime tables and boot tables that contain the idmap.
> This lets us avoid the trampoline dance during initialisation.
> 
> This also means there is no trampoline page mapped, so
> __cpu_reset_hyp_mode() can't call __kvm_hyp_reset() in this page. The good
> news is the idmap is still mapped, so we don't need the trampoline page.
> The bad news is we can't call it directly as the idmap is above
> HYP_PAGE_OFFSET, so its address is masked by kvm_call_hyp.
> 
> Add a function __extended_idmap_trampoline which will branch into
> __kvm_hyp_reset in the idmap, change kvm_hyp_reset_entry() to return
> this address if __kvm_cpu_uses_extended_idmap(). In this case
> __kvm_hyp_reset() will still switch to the boot tables (which are the
> merged tables that were already in use), and branch into the idmap (where
> it already was).
> 
> This fixes boot failures on these systems, where we fail to execute the
> missing trampoline page when tearing down kvm in init_subsystems():
> [    2.508922] kvm [1]: 8-bit VMID
> [    2.512057] kvm [1]: Hyp mode initialized successfully
> [    2.517242] kvm [1]: interrupt-controller at e1140000 IRQ13
> [    2.522622] kvm [1]: timer IRQ3
> [    2.525783] Kernel panic - not syncing: HYP panic:
> [    2.525783] PS:200003c9 PC:0000007ffffff820 ESR:86000005
> [    2.525783] FAR:0000007ffffff820 HPFAR:00000000003ffff0 PAR:0000000000000000
> [    2.525783] VCPU:          (null)
> [    2.525783]
> [    2.547667] CPU: 0 PID: 0 Comm: swapper/0 Tainted: G        W       4.6.0-rc5+ #1
> [    2.555137] Hardware name: Default string Default string/Default string, BIOS ROD0084E 09/03/2015
> [    2.563994] Call trace:
> [    2.566432] [<ffffff80080888d0>] dump_backtrace+0x0/0x240
> [    2.571818] [<ffffff8008088b24>] show_stack+0x14/0x20
> [    2.576858] [<ffffff80083423ac>] dump_stack+0x94/0xb8
> [    2.581899] [<ffffff8008152130>] panic+0x10c/0x250
> [    2.586677] [<ffffff8008152024>] panic+0x0/0x250
> [    2.591281] SMP: stopping secondary CPUs
> [    3.649692] SMP: failed to stop secondary CPUs 0-2,4-7
> [    3.654818] Kernel Offset: disabled
> [    3.658293] Memory Limit: none
> [    3.661337] ---[ end Kernel panic - not syncing: HYP panic:
> [    3.661337] PS:200003c9 PC:0000007ffffff820 ESR:86000005
> [    3.661337] FAR:0000007ffffff820 HPFAR:00000000003ffff0 PAR:0000000000000000
> [    3.661337] VCPU:          (null)
> [    3.661337]
> 
> 
> Reported-by: Will Deacon <[email-masked]>
> Signed-off-by: James Morse <[email-masked]>
> ---
>  arch/arm64/include/asm/kvm_host.h |  3 ++-
>  arch/arm64/kvm/hyp-init.S         |  5 +++++
>  arch/arm64/kvm/hyp/entry.S        | 19 +++++++++++++++++++
>  arch/arm64/kvm/reset.c            | 30 +++++++++++++++++++++++-------
>  4 files changed, 49 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index b6f194d9f6b2..88a34670ddef 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -46,7 +46,8 @@
>  int __attribute_const__ kvm_target_cpu(void);
>  int kvm_reset_vcpu(struct kvm_vcpu *vcpu);
>  int kvm_arch_dev_ioctl_check_extension(long ext);
> -phys_addr_t kvm_hyp_reset_entry(void);
> +unsigned long kvm_hyp_reset_entry(void);
> +void __extended_idmap_trampoline(phys_addr_t boot_pgd, phys_addr_t idmap_start);
>  
>  struct kvm_arch {
>  	/* The VMID generation used for the virt. memory system */
> diff --git a/arch/arm64/kvm/hyp-init.S b/arch/arm64/kvm/hyp-init.S
> index 44ec4cb23ae7..a873a6d8be90 100644
> --- a/arch/arm64/kvm/hyp-init.S
> +++ b/arch/arm64/kvm/hyp-init.S
> @@ -140,6 +140,11 @@ merged:
>  ENDPROC(__kvm_hyp_init)
>  
>  	/*
> +	 * Reset kvm back to the hyp stub. This is the trampoline dance in
> +	 * reverse. If kvm used an extended idmap, __extended_idmap_trampoline
> +	 * calls this code directly in the idmap. In this case switching to the
> +	 * boot tables is a no-op.
> +	 *
>  	 * x0: HYP boot pgd
>  	 * x1: HYP phys_idmap_start
>  	 */
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> index ce9e5e5f28cf..70254a65bd5b 100644
> --- a/arch/arm64/kvm/hyp/entry.S
> +++ b/arch/arm64/kvm/hyp/entry.S
> @@ -164,3 +164,22 @@ alternative_endif
>  
>  	eret
>  ENDPROC(__fpsimd_guest_restore)
> +
> +/*
> + * When using the extended idmap, we don't have a trampoline page we can use
> + * while we switch pages tables during __kvm_hyp_reset. Accessing the idmap
> + * directly would be ideal, but if we're using the extended idmap then the
> + * idmap is located above HYP_PAGE_OFFSET, and the address will be masked by
> + * kvm_call_hyp using kern_hyp_va.
> + *
> + * x0: HYP boot pgd
> + * x1: HYP phys_idmap_start
> + */
> +ENTRY(__extended_idmap_trampoline)
> +	mov	x4, x1
> +	adr_l	x3, __kvm_hyp_reset
> +
> +	/* insert __kvm_hyp_reset()s offset into phys_idmap_start */
> +	bfi	x4, x3, #0, #PAGE_SHIFT
> +	br	x4
> +ENDPROC(__extended_idmap_trampoline)
> diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> index 4062e6dd4cc1..b1ad730e1567 100644
> --- a/arch/arm64/kvm/reset.c
> +++ b/arch/arm64/kvm/reset.c
> @@ -135,12 +135,28 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
>  
>  extern char __hyp_idmap_text_start[];
>  
> -phys_addr_t kvm_hyp_reset_entry(void)
> +unsigned long kvm_hyp_reset_entry(void)
>  {
> -	unsigned long offset;
> -
> -	offset = (unsigned long)__kvm_hyp_reset
> -		 - ((unsigned long)__hyp_idmap_text_start & PAGE_MASK);
> -
> -	return TRAMPOLINE_VA + offset;
> +	if (!__kvm_cpu_uses_extended_idmap()) {
> +		unsigned long offset;
> +
> +		/*
> +		 * Find the address of __kvm_hyp_reset() in the trampoline page.
> +		 * This is present in the running page tables, and the boot page
> +		 * tables, so we call the code here to start the trampoline
> +		 * dance in reverse.
> +		 */
> +		offset = (unsigned long)__kvm_hyp_reset
> +			 - ((unsigned long)__hyp_idmap_text_start & PAGE_MASK);
> +
> +		return TRAMPOLINE_VA + offset;
> +	} else {
> +		/*
> +		 * KVM is running with merged page tables, which don't have the
> +		 * trampoline page mapped. We know the idmap is still mapped,
> +		 * but can't be called into directly. Use
> +		 * __extended_idmap_trampoline to do the call.
> +		 */
> +		return (unsigned long)kvm_ksym_ref(__extended_idmap_trampoline);
> +	}
>  }

This looks correct, and gives me yet another incentive to revisit this
code with a view of keeping the idmap at all times in the EL2 page
tables. This should ensure that (1) the merged page tables become the
only configuration, and (2) that we get rid off of the potential TLB
conflict when swapping TTBR0_EL2 at boot time (intermediate level
caching). In the meantime:

Reviewed-by: Marc Zyngier <[email-masked]>

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny.