Re: [Xen-devel] [PATCH] p2m/ept: pass correct level to atomic_write_ept_entry in ept_invalidate_emt

[Jan Beulich <jbeulich@xxxxxxxx>]

On 29.08.2019 12:26, Roger Pau Monné  wrote:
> On Tue, Aug 27, 2019 at 05:23:33PM +0200, Jan Beulich wrote:
>> On 23.08.2019 07:58,  Tian, Kevin  wrote:
>>>> From: Roger Pau Monne [mailto:roger.pau@xxxxxxxxxx]
>>>> Sent: Tuesday, August 20, 2019 11:38 PM
>>>>
>>>> The level passed to ept_invalidate_emt corresponds to the EPT entry
>>>> passed as the mfn parameter, which is a pointer to an EPT page table,
>>>> hence the entries in that page table will have one level less than the
>>>> parent.
>>>>
>>>> Fix the call to atomic_write_ept_entry to pass the correct level, ie:
>>>> one level less than the parent.
>>>>
>>>> Fixes: 50fe6e73705 ('pvh dom0: add and remove foreign pages')
>>>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>>> ---
>>>> Cc: Jun Nakajima <jun.nakajima@xxxxxxxxx>
>>>> Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
>>>> Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
>>>> Cc: Jan Beulich <jbeulich@xxxxxxxx>
>>>> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>> Cc: Wei Liu <wl@xxxxxxx>
>>>> Cc: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
>>>> Cc: Paul Durrant <paul.durrant@xxxxxxxxxx>
>>>> ---
>>>>   xen/arch/x86/mm/p2m-ept.c | 2 +-
>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/xen/arch/x86/mm/p2m-ept.c b/xen/arch/x86/mm/p2m-ept.c
>>>> index 6b8468c793..23ae6e9da3 100644
>>>> --- a/xen/arch/x86/mm/p2m-ept.c
>>>> +++ b/xen/arch/x86/mm/p2m-ept.c
>>>> @@ -367,7 +367,7 @@ static bool_t ept_invalidate_emt(struct p2m_domain
>>>> *p2m, mfn_t mfn,
>>>>           e.emt = MTRR_NUM_TYPES;
>>>>           if ( recalc )
>>>>               e.recalc = 1;
>>>> -        rc = atomic_write_ept_entry(p2m, &epte[i], e, level);
>>>> +        rc = atomic_write_ept_entry(p2m, &epte[i], e, level - 1);
>>>>           ASSERT(rc == 0);
>>>>           changed = 1;
>>>>       }
>>>
>>> Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>.
>>>
>>> One small comment about readability. What about renaming 'level'
>>> to 'parent_level' in this function?
>>
>> And also taking the opportunity and making it unsigned int?
> 
> I've been thinking about this, and I'm not sure renaming level to
> parent_level is correct, level actually matches the level of the mfn
> also passed as a parameter, and hence it's correct. It's the function
> itself that actually iterates over the page table entries, and hence
> descends one level into the page tables.
> 
> ie: I could add something like ASSERT(level) to make sure no level 0
> entries are passed to the function, but renaming doesn't seem correct
> to me.

Hmm, I'm afraid I've made the change as requested by Kevin while
committing. Personally I think either name is fine, but if Kevin
agrees with your response, then maybe we should undo that adjustment.

Jan

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