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Subject: Re: [PATCH RFC 0/2] KVM: s390: avoid having to enable vm.alloc_pgste
To: Heiko Carstens <heiko.carstens@de.ibm.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Thomas Huth <thuth@redhat.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>
References: <20170529163202.13077-1-david@redhat.com>
 <20170601124651.3e7969ab@mschwideX1> <20170602070210.GA4221@osiris>
From: David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <8d240bf8-9450-d3de-6f38-706bbfa71393@redhat.com>
Date: Fri, 2 Jun 2017 12:54:16 +0200
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On 02.06.2017 09:02, Heiko Carstens wrote:
> On Thu, Jun 01, 2017 at 12:46:51PM +0200, Martin Schwidefsky wrote:
>>> Unfortunately, converting all page tables to 4k pgste page tables is
>>> not possible without provoking various race conditions.
>>
>> That is one approach we tried and was found to be buggy. The point is that
>> you are not allowed to reallocate a page table while a VMA exists that is
>> in the address range of that page table.
>>
>> Another approach we tried is to use an ELF flag on the qemu executable.
>> That does not work either because fs/exec.c allocates and populates the
>> new mm struct for the argument pages before fs/binfmt_elf.c comes into
>> play.
> 
> How about if you would fail the system call within arch_check_elf() if you
> detect that the binary requires pgstes (as indicated by elf flags) and then
> restart the system call?
> 
> That is: arch_check_elf() e.g. would set a thread flag that future mm's
> should be allocated with pgstes. Then do_execve() would cleanup everything
> and return to entry.S. Upon return to userspace we detect this condition
> and simply restart the system call, similar to signals vs -ERESTARTSYS.
> 
> That would make do_execve() cleanup everything and upon reentering it would
> allocate an mm with the pgste flag set.

Cool, I thought that we would not be able to get around having to make a
decision at fork time, but this really looks promising.

Thanks for this idea Heiko!


-- 

Thanks,

David