Re: [PATCH v4 22/33] hostmem-epc: Add the reset interface for EPC backend reset

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/09/21 19:34, Sean Christopherson wrote:
> On Fri, Sep 10, 2021, Paolo Bonzini wrote:
>> On 10/09/21 17:34, Sean Christopherson wrote:
>>> The only other option that comes to mind is a dedicated ioctl().
>>
>> If it is not too restrictive to do it for the whole mapping at once,
>> that would be fine.
> 
> Oooh, rats.  That reminds me of a complication.  If QEMU creates multiple EPC
> sections, e.g. for a vNUMA setup, resetting each section individually will fail
> if the guest did an unclean RESET and a given enclaves has EPC pages from multiple
> sections.  E.g. an SECS in vEPC[X] can have children in vEPC[0..N-1], and all
> those children need to be removed before the SECS can be removed.  Yay SGX!
> 
> There are two options: 1) QEMU has to handle "failure", or 2) the kernel provides
> an ioctl() that takes multiple vEPC fds and handles the SECS dependencies.  #1 is
> probably the least awful option.  For #2, in addition to the kernel having to deal
> with multiple fds, it would also need a second list_head object in each page so
> that it could track which pages failed to be removed.  Using the existing list_head
> would work for now, but it won't work if/when an EPC cgroup is added.
> 
> Note, for #1, QEMU would have to potentially do three passes.
> 
>    1. Remove child pages for a given vEPC.
>    2. Remove SECS for a given vEPC that were pinned by children in the same vEPC.
>    3. Remove SECS for all vEPC that were pinned by children in different vEPC.

It's also possible that QEMU handles failure, but the kernel does two 
passes; then QEMU can just do two passes.  The kernel will overall do 
four passes, but:

1) the second (SECS pinned by children in the same vEPC) would be 
cheaper than a full second pass

2) the fourth would actually do nothing, because there would be no pages 
failing the EREMOV'al.

A hypothetical other SGX client that only uses one vEPC will do the 
right thing with a single pass.

Paolo