Subject: Re: [PATCH][RFC] Replaced tlbilx with tlbwe in the initialization code

From: Diana Craciun <[email-masked]>
Date: Wed, 20 Feb 2013 11:22:12 +0200

On 02/19/2013 09:47 PM, Scott Wood wrote:
> On 02/15/2013 09:16:15 AM, Diana Craciun wrote:
>> On 02/15/2013 02:11 AM, Benjamin Herrenschmidt wrote:
>>> On Thu, 2013-02-14 at 14:56 +0200, Diana Craciun wrote:
>>>> From: Diana Craciun <[email-masked]>
>>>>
>>>> On Freescale e6500 cores EPCR[DGTMI] controls whether guest
>>>> supervisor
>>>> state can execute TLB management instructions. If EPCR[DGTMI]=0
>>>> tlbwe and tlbilx are allowed to execute normally in the guest state.
>>>>
>>>> A hypervisor may choose to virtualize TLB1 and for this purpose it
>>>> may use IPROT to protect the entries for being invalidated by the
>>>> guest. However, because tlbwe and tlbilx execution in the guest
>>>> state
>>>> are sharing the same bit, it is not possible to have a scenario
>>>> where
>>>> tlbwe is allowed to be executed in guest state and tlbilx traps.
>>>> When
>>>> guest TLB management instructions are allowed to be executed in
>>>> guest
>>>> state the guest cannot use tlbilx to invalidate TLB1 guest entries.
>>> Sorry, I don't understand the explanation... can you be more
>>> detailed ?
>> TLB1 supports huge page sizes. The guest may see the memory as
>> contiguous but it sees the guest physical memory as presented by the
>> hypervisor. In reality the real physical memory may be fragmented. In
>> this case the hypervisor can add more than one TLB1 entry for one
>> guest request and the hypervisor will keep track of all fragments.
>> When the guest performs a tlbilx, the hypervisor will correctly
>> invalidate all the corresponding fragments because both tlbwe and
>> tlbilx trap and has full control of tlb management instructions
>> targeting TLB1.
>>
>> For e6500 a single bit controls if tlbwe and tlbilx trap to the
>> Hypervisor. tlbwe targeting TLB1 always traps. But if we want to use
>> LRAT for TLB0, we have to configure tlbwe (targeting TLB 0) to go
>> directly to the guest. But in this case tlbilx (which is targeting
>> both TLBs) will never trap.
>>
>> If the tlbilx does not trap, the guest can invalidate only one of
>> (possible more) fragments and furthermore the synchronization between
>> what entries the hypervisor thinks there are in the TLB1 and what are
>> the actual entries is lost.
> This patch addresses boot-time invalidations only.  How will you handle
> hugetlb invalidations (or indirect entry invalidations, once that
> becomes supported)?
>
> -Scott

I will not handle them. This patch offers the possibility to run Linux 
under hypervisor without using hugetlb or indirect entries (of course in 
case when we configure tlb management instructions to go to the guest 
because otherwise it works)

If indirect entries are supported most likely we will configure tlbilx 
and tlbwe to trap. In this case LRAT will be still used through the page 
table walk mechanism.

Diana