Re: [Xen-devel] [PATCH RFC v2 2/4] x86/mem_access: mem_access and mem_event changes to support PV domains

["Aravindh Puthiyaparambil (aravindp)" <aravindp@xxxxxxxxx>]

>-----Original Message-----
>From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>Sent: Friday, July 25, 2014 12:23 AM
>To: Aravindh Puthiyaparambil (aravindp)
>Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; KeirFraser; Tim Deegan
>Subject: RE: [PATCH RFC v2 2/4] x86/mem_access: mem_access and
>mem_event changes to support PV domains
>
>>>> On 25.07.14 at 01:52, <aravindp@xxxxxxxxx> wrote:
>>> > +static int mem_access_set_default(struct domain *d, uint64_t
>>>*start_page,
>>>> +                           xenmem_access_t access) {
>>>> +    struct p2m_domain *p2m = p2m_get_hostp2m(d);
>>>> +    struct page_info *page;
>>>> +    struct page_list_head head;
>>>> +    p2m_access_t a;
>>>> +    int rc = 0, ctr = 0;
>>>> +
>>>> +    if ( !is_pv_domain(d) )
>>>> +        return -ENOSYS;
>>>> +
>>>> +    ASSERT(shadow_mode_enabled(d));
>>>> +
>>>> +    rc = p2m_convert_xenmem_access(p2m, access, &a);
>>>> +    if ( rc != 0 )
>>>> +        return rc;
>>>> +
>>>> +    /*
>>>> +     * For PV domains we only support r, rw, rx, rx2rw and rwx access
>>>> +     * permissions
>>>> +     */
>>>> +    switch ( a )
>>>> +    {
>>>> +    case p2m_access_n:
>>>> +    case p2m_access_w:
>>>> +    case p2m_access_x:
>>>> +    case p2m_access_wx:
>>>> +    case p2m_access_n2rwx:
>>>> +        return -EINVAL;
>>>> +    default:
>>>> +        break;
>>>> +    }
>>>> +
>>>> +    paging_lock_recursive(d);
>>>> +
>>>> +    if ( *start_page )
>>>> +    {
>>>> +        head.next = (struct page_info *)*start_page;
>>>
>>>What guarantees that the continuation page is still on d->page_list, or that
>>>now other page got inserted ahead of it? And anyway you're iterating the
>list
>>>without holding d->page_alloc_lock.
>>
>> Good point. Should I grab the lock and release it only when the hypercall
>> completes?
>
>If "completes" means after any eventual continuation, then you
>should be able to answer this with "no" yourself. If "completes" is

Oh no, I really worded my question badly.

>meant only up to the next continuation, then this wouldn't help
>you anyway. 

Yes, that is what I meant. And yes, I realize it won't be of any help.

>IOW this needs a more sophisticated solution, or

OK, I will look in to reusing the p2m-pt implementation.

>you need to restrict the memory size of guests that can be subject
>to mem-access handling.

If I was to stick to using the shadow_flags, what should the memory size 
restriction be so as to not have a hypercall continuation? 

Thanks,
Aravindh


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