On 06/14/2012 06:40 AM, Marcelo Tosatti wrote:
> On Tue, May 29, 2012 at 02:50:32PM +0800, Xiao Guangrong wrote:
>> If the the present bit of page fault error code is set, it indicates
>> the shadow page is populated on all levels, it means what we do is
>> only modify the access bit which can be done out of mmu-lock
>>
>> Currently, in order to simplify the code, we only fix the page fault
>> caused by write-protect on the fast path
>>
>> Signed-off-by: Xiao Guangrong <[email protected]>
>> ---
>> arch/x86/kvm/mmu.c | 126
>> +++++++++++++++++++++++++++++++++++++++++++++++-----
>> 1 files changed, 114 insertions(+), 12 deletions(-)
>>
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index 150c5ad..d6101a8 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -445,6 +445,11 @@ static bool __check_direct_spte_mmio_pf(u64 spte)
>> }
>> #endif
>>
>> +static bool spte_can_be_writable(u64 spte)
>> +{
>> + return !(~spte & (SPTE_HOST_WRITEABLE | SPTE_MMU_WRITEABLE));
>> +}
>> +
>
> spte_is_locklessly_modifiable(). Its easy to confuse
> "spte_can_be_writable" with different things.
>
Yes. Will update it.
>> static bool spte_has_volatile_bits(u64 spte)
>> {
>> if (!shadow_accessed_mask)
>> @@ -454,7 +459,7 @@ static bool spte_has_volatile_bits(u64 spte)
>> return false;
>>
>> if ((spte & shadow_accessed_mask) &&
>> - (!is_writable_pte(spte) || (spte & shadow_dirty_mask)))
>> + (!spte_can_be_writable(spte) || (spte & shadow_dirty_mask)))
>> return false;
>
> mmu_spte_update is handling several different cases. Please rewrite
> it, add a comment on top of it (or spread comments on top of each
> significant code line) with all cases it is handling (also recheck it
> regarding new EPT accessed/dirty bits code).
>
Okay.
> For one thing, if spte can be updated locklessly the update must be
> atomic:
>
> if spte can be locklessly updated
> read-and-modify must be atomic.
Actually, i did it in the v5, Avi has some comments on that. Please
see https://lkml.org/lkml/2012/5/24/55
What the reason we should locklessly update spte here? So far i know
is for volatile bit lost and getting a stable is_writable_spte()?
But this two cases can be avoided by using spte_can_be_writable(spte)
instead of is_writable_pte(spte), right?
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