Re: [PATCH RFC 0/4] 5-level EPT

[Paolo Bonzini]

On 17/01/2017 03:18, Li, Liang Z wrote:
>> On 29/12/2016 10:25, Liang Li wrote:
>>> x86-64 is currently limited physical address width to 46 bits, which
>>> can support 64 TiB of memory. Some vendors require to support more for
>>> some use case. Intel plans to extend the physical address width to
>>> 52 bits in some of the future products.
>>>
>>> The current EPT implementation only supports 4 level page table, which
>>> can support maximum 48 bits physical address width, so it's needed to
>>> extend the EPT to 5 level to support 52 bits physical address width.
>>>
>>> This patchset has been tested in the SIMICS environment for 5 level
>>> paging guest, which was patched with Kirill's patchset for enabling
>>> 5 level page table, with both the EPT and shadow page support. I just
>>> covered the booting process, the guest can boot successfully.
>>>
>>> Some parts of this patchset can be improved. Any comments on the
>>> design or the patches would be appreciated.
>>
>> I will review the patches.  They seem fairly straightforward.
>>
>> However, I am worried about the design of the 5-level page table feature
>> with respect to migration.
>>
>> Processors that support the new LA57 mode can write 57-canonical/48-
>> noncanonical linear addresses to some registers even when LA57 mode is
>> inactive.  This is true even of unprivileged instructions, in particular
>> WRFSBASE/WRGSBASE.
>>
>> This is fairly bad because, if a guest performs such a write (because of a bug
>> or because of malice), it will not be possible to migrate the virtual machine to
>> a machine that lacks LA57 mode.
>>
>> Ordinarily, hypervisors trap CPUID to hide features that are only present in
>> some processors of a heterogeneous cluster, and the hypervisor also traps
>> for example CR4 writes to prevent enabling features that were masked away.
>> In this case, however, the only way for the hypervisor to prevent the write
>> would be to run the guest with
>> CR4.FSGSBASE=0 and trap all executions of WRFSBASE/WRGSBASE.  This
>> might have negative effects on performance for workloads that use the
>> instructions.
>>
>> Of course, this is a problem even without your patches.  However, I think it
>> should be addressed first.  I am seriously thinking of blacklisting FSGSBASE
>> completely on LA57 machines until the above is fixed in hardware.
>>
>> Paolo
> 
> The issue has already been forwarded to the hardware guys, still waiting for the feedback.

Going to review this now.  Any news?

Paolo