Re: [PATCH v4 6/7] xen/riscv: page table handling

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.08.2024 15:18, oleksii.kurochko@xxxxxxxxx wrote:
> On Tue, 2024-08-13 at 12:31 +0200, Jan Beulich wrote:
>>> + * Sanity check of the entry
>>> + * mfn is not valid and we are not populating page table. This
>>> means
>>
>> How does this fit with ...
>>
>>> + * we either modify entry or remove an entry.
>>> + */
>>> +static bool pt_check_entry(pte_t entry, mfn_t mfn, unsigned int
>>> flags)
>>> +{
>>> +    /* Sanity check when modifying an entry. */
>>> +    if ( (flags & PTE_VALID) && mfn_eq(mfn, INVALID_MFN) )
>>
>> ... the MFN check here? And why is (valid,INVALID_MFN) an indication
>> of a modification?
> Because as mentioned here:
> ```
> /*
>  * If `mfn` equals `INVALID_MFN`, it indicates that the following page
> table
>  * update operation might be related to either populating the table,
>  * destroying a mapping, or modifying an existing mapping.
>  */
> static int pt_update(unsigned long virt,
> ```

That's in the description of another function. How would one know that
the rules on (mfn,flags) tuples there would apply here as well, without
you saying so explicitly? It may not be necessary to repeat the other
comment, but at least you want to reference it.

> And so if requested flags are PTE_VALID ( present ) and mfn=INVALID it
> will mean that we are going to modify an entry.
> 
> 
>> But then ...
>>
>>> +    {
>>> +        /* We don't allow modifying an invalid entry. */
>>> +        if ( !pte_is_valid(entry) )
>>> +        {
>>> +            printk("Modifying invalid entry is not allowed.\n");
>>> +            return false;
>>> +        }
>>
>> ... I also don't understand what this is about. IOW I'm afraid I'm
>> still confused about the purpose of this function as well as the
>> transitions you want to permit / reject. 
> In the case if the caller call modify_xen_mappings() on a region that
> doesn't exist.

Perhaps. What I think is missing is a clear statement somewhere to describe
what the various combinations of (mfn,flags) mean, in terms of the operation
to be carried out. This may then also help with ...

>> I wonder whether the
>> flags & PTE_VALID and pte_is_valid(entry) aren't in need of swapping.
> I am not sure that I understand what you mean.

... this: It's hard to see what cannot be understood about my earlier
comment. In the code commented on you have a flags & PTE_VALID check and a
pte_is_valid(entry) one. I'm wondering whether the two simply are the wrong
way round.

>>> +/* Update an entry at the level @target. */
>>> +static int pt_update_entry(mfn_t root, unsigned long virt,
>>> +                           mfn_t mfn, unsigned int target,
>>> +                           unsigned int flags)
>>> +{
>>> +    int rc;
>>> +    unsigned int level = HYP_PT_ROOT_LEVEL;
>>> +    pte_t *table;
>>> +    /*
>>> +     * The intermediate page tables are read-only when the MFN is
>>> not valid
>>> +     * and we are not populating page table.
>>
>> The way flags are handled in PTEs, how can page tables be read-only?
> This is not needed for everyone case. In case of entry removing an
> intermediate page table should be created in case when the user is
> trying to remove a mapping that doesn't exist.

I don't follow: For one, how is this related to "read-only"-ness? And
then, why would any kind of removal, whether of a present or non-
present mapping, ever result in page tables being created?

>>> +     * This means we either modify permissions or remove an entry.
>>
>> From all I can determine we also get here when making brand new
>> entries.
>> What am I overlooking?
> Yes, but in this case an intermediate page tables should be read_only,
> so alloc_only will be true and it will be allowed to create new
> intermediate page table.

Hmm, so instead of "read-only" do you maybe mean page tables are not
supposed to be modified? There's a difference here: When they're
read-only, you can't write to them (or a fault will result). Whereas
when in principle they can be modified, there still may be a rule
saying "in this case they shouldn't be altered".

Jan