Re: [PATCH Part2 RFC v4 07/40] x86/sev: Split the physmap when adding the page in RMP table

[Brijesh Singh]

On 7/15/21 5:01 PM, Sean Christopherson wrote:
> On Thu, Jul 15, 2021, Brijesh Singh wrote:
>>
>> On 7/15/21 1:39 PM, Sean Christopherson wrote:
>>> On Thu, Jul 15, 2021, Brijesh Singh wrote:
>>>> The memfd_secrets uses the set_direct_map_{invalid,default}_noflush() and it
>>>> is designed to remove/add the present bit in the direct map. We can't use
>>>> them, because in our case the page may get accessed by the KVM (e.g
>>>> kvm_guest_write, kvm_guest_map etc).
>>> But KVM should never access a guest private page, i.e. the direct map should
>>> always be restored to PRESENT before KVM attempts to access the page.
>>>
>> Yes, KVM should *never* access the guest private pages. So, we could
>> potentially enhance the RMPUPDATE() to check for the assigned and act
>> accordingly.
>>
>> Are you thinking something along the line of this:
>>
>> int rmpupdate(struct page *page, struct rmpupdate *val)
>> {
>> 	...
>> 	
>> 	/*
>> 	 * If page is getting assigned in the RMP entry then unmap
>> 	 * it from the direct map before its added in the RMP table.
>> 	 */
>> 	if (val.assigned)
>> 		set_direct_map_invalid_noflush(page_to_virt(page), 1);
>>
>> 	...
>>
>> 	/*
>> 	 * If the page is getting unassigned then restore the mapping
>> 	 * in the direct map after its removed from the RMP table.
>> 	 */
>> 	if (!val.assigned)
>> 		set_direct_map_default_noflush(page_to_virt(page), 1);
>> 	
>> 	...
>> }
> Yep.
>
> However, looking at the KVM usage, rmpupdate() appears to be broken.  When
> handling a page state change, the guest can specify a 2mb page.  In that case,
> rmpupdate() will be called once for a 2mb page, but this flow assumes a single
> 4kb page.  The current code works because set_memory_4k() will cause the entire
> 2mb page to be shattered, but it's technically wrong and switching to the above
> would cause problems.


Yep, this was just an example to make sure I am able to follow you
correctly. In the actual patch I am going to read the pagesize from the
RMPUPDATE structure and  calculated npages for the
set_direct_map_default(...). As you said it was not needed in the case
of set_memory_4k() because the function force splits the large page.
Whereas with set_direct_map_default(), it first checks whether the split
is required, if not, then skip and update the attributes.

-Brijesh