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* Re: Re: [PATCH] KVM:arm/arm64: dcache need be coherent unconditionally
@ 2018-03-10  3:48 ` peng.hao2 at zte.com.cn
  0 siblings, 0 replies; 3+ messages in thread
From: peng.hao2 @ 2018-03-10  3:48 UTC (permalink / raw)
  To: marc.zyngier; +Cc: cdall, linux-kernel, linux-arm-kernel, kvmarm


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>On 09/03/18 22:15, Peng Hao wrote:
>> For emulation devices just like vga, keeping coherent dcache between
>> guest and host timely is needed.
>> Now the display of vnc-viewer will not update continuously and the
>> patch can fix up.
>> 
>> Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
>> ---
>>  virt/kvm/arm/mmu.c | 6 ++----
>>  1 file changed, 2 insertions(+), 4 deletions(-)
>> 
>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
>> index ec62d1c..4a28395e 100644
>> --- a/virt/kvm/arm/mmu.c
>> +++ b/virt/kvm/arm/mmu.c
>> @@ -1416,8 +1416,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>              kvm_set_pfn_dirty(pfn);
>>          }
>>  
>> -        if (fault_status != FSC_PERM)
>> -            clean_dcache_guest_page(pfn, PMD_SIZE);
>> +        clean_dcache_guest_page(pfn, PMD_SIZE);
>>  
>>          if (exec_fault) {
>>              new_pmd = kvm_s2pmd_mkexec(new_pmd);
>> @@ -1438,8 +1437,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>              mark_page_dirty(kvm, gfn);
>>          }
>>  
>> -        if (fault_status != FSC_PERM)
>> -            clean_dcache_guest_page(pfn, PAGE_SIZE);
>> +        clean_dcache_guest_page(pfn, PAGE_SIZE);
>>  
>>          if (exec_fault) {
>>              new_pte = kvm_s2pte_mkexec(new_pte);
>> 

>I'm sorry, but I have to NAK this.

>You're papering over the fundamental issue that you're accessing a
>cacheable alias of a non cacheable memory. The architecture is very
>clear about why this doesn't work, and KVM implements the architecture.

I find that I  just encounter the problem after  the commit '15303ba5d1cd9b28d03a980456c0978c0ea3b208 " .
The commit contains "icache invalidation optimizations, improving VM startup time",it changes
from unconditionally calling coherent_cache_guest_page(including dcache handle) to conditionally calling clean_dcache_guest_page.
 I trace the display of vnc abnormally and find it generate data abort in vga
address region with FSC_PERM,and it will not call clean_dcache_guest_page . So I think  should recover to uncontionally calling clean_dcache_guest_page.

Thanks.
>If you want this to work, map your VGA device as cacheable, add cache
>maintenance to QEMU, or use another frame-buffer emulation that doesn't
>require such a gack.

>Thanks,

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

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^ permalink raw reply	[flat|nested] 3+ messages in thread
* Re: [PATCH] KVM:arm/arm64: dcache need be coherent unconditionally
@ 2018-03-09 14:21 Marc Zyngier
  2018-03-10  3:23 ` peng.hao2
  0 siblings, 1 reply; 3+ messages in thread
From: Marc Zyngier @ 2018-03-09 14:21 UTC (permalink / raw)
  To: Peng Hao, christoffer.dall; +Cc: linux-arm-kernel, kvmarm, linux-kernel

On 09/03/18 22:15, Peng Hao wrote:
> For emulation devices just like vga, keeping coherent dcache between
> guest and host timely is needed.
> Now the display of vnc-viewer will not update continuously and the
> patch can fix up.
> 
> Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
> ---
>  virt/kvm/arm/mmu.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index ec62d1c..4a28395e 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -1416,8 +1416,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  			kvm_set_pfn_dirty(pfn);
>  		}
>  
> -		if (fault_status != FSC_PERM)
> -			clean_dcache_guest_page(pfn, PMD_SIZE);
> +		clean_dcache_guest_page(pfn, PMD_SIZE);
>  
>  		if (exec_fault) {
>  			new_pmd = kvm_s2pmd_mkexec(new_pmd);
> @@ -1438,8 +1437,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  			mark_page_dirty(kvm, gfn);
>  		}
>  
> -		if (fault_status != FSC_PERM)
> -			clean_dcache_guest_page(pfn, PAGE_SIZE);
> +		clean_dcache_guest_page(pfn, PAGE_SIZE);
>  
>  		if (exec_fault) {
>  			new_pte = kvm_s2pte_mkexec(new_pte);
> 

I'm sorry, but I have to NAK this.

You're papering over the fundamental issue that you're accessing a
cacheable alias of a non cacheable memory. The architecture is very
clear about why this doesn't work, and KVM implements the architecture.

If you want this to work, map your VGA device as cacheable, add cache
maintenance to QEMU, or use another frame-buffer emulation that doesn't
require such a gack.

Thanks,

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

^ permalink raw reply	[flat|nested] 3+ messages in thread
end of thread, other threads:[~2018-03-10  3:48 UTC | newest]

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2018-03-10  3:48 Re: [PATCH] KVM:arm/arm64: dcache need be coherent unconditionally peng.hao2
2018-03-10  3:48 ` peng.hao2 at zte.com.cn
  -- strict thread matches above, loose matches on Subject: below --
2018-03-09 14:21 Marc Zyngier
2018-03-10  3:23 ` peng.hao2

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