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

From: [email-masked] (Marc Zyngier)
Date: Tue, 3 May 2016 09:41:22 +0100

On 03/05/16 09:36, Christoffer Dall wrote:
> On Mon, May 02, 2016 at 01:05:35PM +0100, Marc Zyngier wrote:
>> 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!  Will, should this go via the kvmarm tree or the arm64 tree?

I believe this issue only exists with the hibernation patches, so I
suggest we let Will handle this though the arm64 tree together with the
rest of this series.

Thanks,

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