linux-kernel - Re: [PATCH 09/15] KVM: selftests: Move per-VM GPA into perf_test

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Date:   Thu, 11 Feb 2021 09:33:25 -0800
From:   Ben Gardon <bgardon@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Yanan Wang <wangyanan55@...wei.com>,
        Andrew Jones <drjones@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Aaron Lewis <aaronlewis@...gle.com>
Subject: Re: [PATCH 09/15] KVM: selftests: Move per-VM GPA into perf_test_args

On Thu, Feb 11, 2021 at 7:58 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Thu, Feb 11, 2021, Paolo Bonzini wrote:
> > On 11/02/21 02:56, Sean Christopherson wrote:
> > > > > +       pta->gpa = (vm_get_max_gfn(vm) - guest_num_pages) * pta->guest_page_size;
> > > > > +       pta->gpa &= ~(pta->host_page_size - 1);
> > > > Also not related to this patch, but another case for align.
> > > >
> > > > >          if (backing_src == VM_MEM_SRC_ANONYMOUS_THP ||
> > > > >              backing_src == VM_MEM_SRC_ANONYMOUS_HUGETLB)
> > > > > -               guest_test_phys_mem &= ~(KVM_UTIL_HUGEPAGE_ALIGNMENT - 1);
> > > > > -
> > > > > +               pta->gpa &= ~(KVM_UTIL_HUGEPAGE_ALIGNMENT - 1);
> > > > also align
> > > >
> > > > >   #ifdef __s390x__
> > > > >          /* Align to 1M (segment size) */
> > > > > -       guest_test_phys_mem &= ~((1 << 20) - 1);
> > > > > +       pta->gpa &= ~((1 << 20) - 1);
> > > > And here again (oof)
> > >
> > > Yep, I'll fix all these and the align() comment in v2.
> >
> > This is not exactly align in fact; it is x & ~y rather than (x + y) & ~y.
> > Are you going to introduce a round-down macro or is it a bug?  (I am
> > lazy...).
>
> Good question.  I, too, was lazy.  I didn't look at the guts of align() when I
> moved it, and I didn't look closely at Ben's suggestion.  I'll take a closer
> look today and make sure everything is doing what it's supposed to do.

Ooh, great point Paolo, that helper is indeed rounding up. My comment
in patch #2 was totally wrong. I forgot anyone would ever want to
round up. :/
My misunderstanding and the above use cases are probably good evidence
that it would be helpful to have both align_up and align_down helpers.