linux-kernel - Re: [PATCH RFC 0/2] KVM: s390: avoid having to enable vm.alloc

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Date:   Thu, 8 Jun 2017 15:17:24 +0200
From:   Heiko Carstens <heiko.carstens@...ibm.com>
To:     Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Thomas Huth <thuth@...hat.com>,
        Andreas Krebbel <krebbel@...ux.vnet.ibm.com>
Subject: Re: [PATCH RFC 0/2] KVM: s390: avoid having to enable vm.alloc_pgste

On Thu, Jun 08, 2017 at 01:24:01PM +0200, Martin Schwidefsky wrote:
> On Thu, 8 Jun 2017 08:25:31 +0200
> Heiko Carstens <heiko.carstens@...ibm.com> wrote:
> > It would be more consistent, since right now a 32-bit ELF file with
> > PT_S390_REQUEST_PGSTE will be exectuted, but the page tables won't have any
> > pgstes. That's sort of odd, isn't it? And that later on it won't be able to
> > create a virtual machine because our current implementation doesn't allow
> > that for compat tasks is sort of unrelated.
> > But anyway, I'll leave that up to you, it doesn't really matter.
> 
> Actually the code will be less complex if we add PT_S390_REQUEST_PGSTE for
> 32-bit ELF files as well. It does not make sense to define the segment for
> a compat process as KVM won't work but you get what you ask for..
> 
> This looks like this:
> 
> #define arch_elf_pt_proc(ehdr, phdr, elf, interp, state)        \
> ({                                                              \
>         int _rc = 0;                                            \
>         if (phdr->p_type == PT_S390_REQUEST_PGSTE &&            \
>             !page_table_allocate_pgste &&                       \
>             !test_thread_flag(TIF_REQUEST_PGSTE)) {             \
>                 set_thread_flag(TIF_REQUEST_PGSTE);             \
>                 set_pt_regs_flag(task_pt_regs(current),         \
>                                  PIF_SYSCALL_RESTART);          \
>                 _rc = -EAGAIN;                                  \
>         }                                                       \
>         _rc;                                                    \
> })
> 
> phdr is a (struct elf_phd *) which is either define to a a (struct elf64_phdr *)
> or a (struct elf32_phdr *). The check works in both cases.

Yes, that makes sense.