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Date:   Thu, 21 Jul 2022 21:16:10 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Vishal Annapurve <vannapurve@google.com>
Cc:     x86 <x86@kernel.org>, kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
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        Paolo Bonzini <pbonzini@redhat.com>,
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        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        dave.hansen@linux.intel.com, "H . Peter Anvin" <hpa@zytor.com>,
        shauh@kernel.org, yang.zhong@intel.com, drjones@redhat.com,
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        Aaron Lewis <aaronlewis@google.com>, wei.w.wang@intel.com,
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        Peter Gonda <pgonda@google.com>,
        "Nikunj A. Dadhania" <nikunj@amd.com>,
        Austin Diviness <diviness@google.com>
Subject: Re: [RFC V2 PATCH 2/8] selftests: kvm: Add a basic selftest to test
 private memory
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References: <20220511000811.384766-1-vannapurve@google.com>
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 <CAGtprH9BQkcJcpp=uEJJLwM-Z=cW9rsJ7iVKbjv_gisVj8EWGQ@mail.gmail.com>
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On Thu, Jul 21, 2022, Vishal Annapurve wrote:
> On Wed, Jul 20, 2022 at 4:03 PM Sean Christopherson <seanjc@google.com> wrote:
> > ...
> > > + * which doesn't handle global offset table updates. Calling standard libc
> > > + * functions would normally result in referring to the global offset table.
> > > + * Adding O1 here seems to prohibit compiler from replacing the memory
> > > + * operations with standard libc functions such as memset.
> > > + */
> >
> > Eww.  We should either fix kvm_vm_elf_load() or override the problematic libc
> > variants.  Playing games with per-function attributes is not maintainable.
> >
> 
> I will try to spend more time on how kvm_vm_elf_load can be modified
> to handle GOT fixups in different scenarios including
> statically/dynamically linked sefltest binaries as I currently recall
> limited information here.
> 
> But modifying kvm_vm_elf_load to fixup GOT entries will be
> insufficient as guest VM code (possibly whole selftest binary) will
> need to be compiled with flags that allow memset/memcpy
> implementations to work with specific guest VM configurations e.g. AVX
> extension. Same concern is outlined in
> https://elixir.bootlin.com/linux/latest/source/tools/testing/selftests/kvm/lib/x86_64/svm.c#L64.
> 
> Would it be ok to maintain selftest binary compilation flags based on
> guest VM configurations?

No, we should instead define/implement versions of memset/memcpy that are
guaranteed to be guest-friendly, either explicitly by selftests are implicitly
by compiler builtins, e.g. see arch/x86/boot/string.h.