On 18.07.2013, at 10:25, Bhushan Bharat-R65777 wrote:
>
>
>> -----Original Message-----
>> From: Bhushan Bharat-R65777
>> Sent: Thursday, July 18, 2013 1:53 PM
>> To: '"“tiejun.chen”"'
>> Cc: [email protected]; [email protected]; [email protected]; Wood Scott-
>> B07421
>> Subject: RE: [PATCH 2/2] kvm: powerpc: set cache coherency only for kernel
>> managed pages
>>
>>
>>
>>> -----Original Message-----
>>> From: "“tiejun.chen”" [mailto:[email protected]]
>>> Sent: Thursday, July 18, 2013 1:52 PM
>>> To: Bhushan Bharat-R65777
>>> Cc: [email protected]; [email protected]; [email protected]; Wood
>>> Scott-
>>> B07421
>>> Subject: Re: [PATCH 2/2] kvm: powerpc: set cache coherency only for
>>> kernel managed pages
>>>
>>> On 07/18/2013 04:08 PM, Bhushan Bharat-R65777 wrote:
>>>>
>>>>
>>>>> -----Original Message-----
>>>>> From: [email protected]
>>>>> [mailto:[email protected]] On Behalf Of "“tiejun.chen”"
>>>>> Sent: Thursday, July 18, 2013 1:01 PM
>>>>> To: Bhushan Bharat-R65777
>>>>> Cc: [email protected]; [email protected]; [email protected];
>>>>> Wood
>>>>> Scott-
>>>>> B07421
>>>>> Subject: Re: [PATCH 2/2] kvm: powerpc: set cache coherency only for
>>>>> kernel managed pages
>>>>>
>>>>> On 07/18/2013 03:12 PM, Bhushan Bharat-R65777 wrote:
>>>>>>
>>>>>>
>>>>>>> -----Original Message-----
>>>>>>> From: "“tiejun.chen”" [mailto:[email protected]]
>>>>>>> Sent: Thursday, July 18, 2013 11:56 AM
>>>>>>> To: Bhushan Bharat-R65777
>>>>>>> Cc: [email protected]; [email protected]; [email protected];
>>>>>>> Wood
>>>>>>> Scott- B07421; Bhushan Bharat-R65777
>>>>>>> Subject: Re: [PATCH 2/2] kvm: powerpc: set cache coherency only
>>>>>>> for kernel managed pages
>>>>>>>
>>>>>>> On 07/18/2013 02:04 PM, Bharat Bhushan wrote:
>>>>>>>> If there is a struct page for the requested mapping then it's
>>>>>>>> normal DDR and the mapping sets "M" bit (coherent, cacheable)
>>>>>>>> else this is treated as I/O and we set "I + G" (cache
>>>>>>>> inhibited,
>>>>>>>> guarded)
>>>>>>>>
>>>>>>>> This helps setting proper TLB mapping for direct assigned device
>>>>>>>>
>>>>>>>> Signed-off-by: Bharat Bhushan <[email protected]>
>>>>>>>> ---
>>>>>>>> arch/powerpc/kvm/e500_mmu_host.c | 17 ++++++++++++-----
>>>>>>>> 1 files changed, 12 insertions(+), 5 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/arch/powerpc/kvm/e500_mmu_host.c
>>>>>>>> b/arch/powerpc/kvm/e500_mmu_host.c
>>>>>>>> index 1c6a9d7..089c227 100644
>>>>>>>> --- a/arch/powerpc/kvm/e500_mmu_host.c
>>>>>>>> +++ b/arch/powerpc/kvm/e500_mmu_host.c
>>>>>>>> @@ -64,13 +64,20 @@ static inline u32
>>>>>>>> e500_shadow_mas3_attrib(u32 mas3, int
>>>>>>> usermode)
>>>>>>>> return mas3;
>>>>>>>> }
>>>>>>>>
>>>>>>>> -static inline u32 e500_shadow_mas2_attrib(u32 mas2, int
>>>>>>>> usermode)
>>>>>>>> +static inline u32 e500_shadow_mas2_attrib(u32 mas2, pfn_t pfn)
>>>>>>>> {
>>>>>>>> + u32 mas2_attr;
>>>>>>>> +
>>>>>>>> + mas2_attr = mas2 & MAS2_ATTRIB_MASK;
>>>>>>>> +
>>>>>>>> + if (!pfn_valid(pfn)) {
>>>>>>>
>>>>>>> Why not directly use kvm_is_mmio_pfn()?
>>>>>>
>>>>>> What I understand from this function (someone can correct me) is
>>>>>> that it
>>>>> returns "false" when the page is managed by kernel and is not
>>>>> marked as RESERVED (for some reason). For us it does not matter
>>>>> whether the page is reserved or not, if it is kernel visible page then it
>> is DDR.
>>>>>>
>>>>>
>>>>> I think you are setting I|G by addressing all mmio pages, right? If
>>>>> so,
>>>>>
>>>>> KVM: direct mmio pfn check
>>>>>
>>>>> Userspace may specify memory slots that are backed by mmio
>>>>> pages rather than
>>>>> normal RAM. In some cases it is not enough to identify these
>>>>> mmio
>>> pages
>>>>> by pfn_valid(). This patch adds checking the PageReserved as well.
>>>>
>>>> Do you know what are those "some cases" and how checking
>>>> PageReserved helps in
>>> those cases?
>>>
>>> No, myself didn't see these actual cases in qemu,too. But this should
>>> be chronically persistent as I understand ;-)
>>
>> Then I will wait till someone educate me :)
>
> The reason is , kvm_is_mmio_pfn() function looks pretty heavy and I do not
> want to call this for all tlbwe operation unless it is necessary.
It certainly does more than we need and potentially slows down the fast path
(RAM mapping). The only thing it does on top of "if (pfn_valid())" is to check
for pages that are declared reserved on the host. This happens in 2 cases:
1) Non cache coherent DMA
2) Memory hot remove
The non coherent DMA case would be interesting, as with the mechanism as it is
in place in Linux today, we could potentially break normal guest operation if
we don't take it into account. However, it's Kconfig guarded by:
depends on 4xx || 8xx || E200 || PPC_MPC512x || GAMECUBE_COMMON
default n if PPC_47x
default y
so we never hit it with any core we care about ;).
Memory hot remove does not exist on e500 FWIW, so we don't have to worry about
that one either.
Which means I think it's fine to slim this whole thing down to only check for
pfn_valid(), as the code does in this patch. It would however be very useful to
have a comment there that explains why it's safe to do so.
Alex
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