linux-kernel - Re: [PATCH v4 09/10] KVM: selftests: Stop memslot creation in KVM internal memslot region

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Message-ID: <CANgfPd81ocW0O4nqSnfBwEn_V-ZzY_WJ=TvymCBfy_w5==GxmQ@mail.gmail.com>
Date:   Mon, 27 Jan 2020 09:28:01 -0800
From:   Ben Gardon <bgardon@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Cannon Matthews <cannonmatthews@...gle.com>,
        Peter Xu <peterx@...hat.com>,
        Andrew Jones <drjones@...hat.com>,
        Peter Shier <pshier@...gle.com>,
        Oliver Upton <oupton@...gle.com>
Subject: Re: [PATCH v4 09/10] KVM: selftests: Stop memslot creation in KVM
 internal memslot region

On Sat, Jan 25, 2020 at 1:37 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 24/01/20 19:41, Ben Gardon wrote:
> > On Fri, Jan 24, 2020 at 12:58 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
> >>
> >> On 23/01/20 19:04, Ben Gardon wrote:
> >>> KVM creates internal memslots covering the region between 3G and 4G in
> >>> the guest physical address space, when the first vCPU is created.
> >>> Mapping this region before creation of the first vCPU causes vCPU
> >>> creation to fail. Prohibit tests from creating such a memslot and fail
> >>> with a helpful warning when they try to.
> >>>
> >>> Signed-off-by: Ben Gardon <bgardon@...gle.com>
> >>> ---
> >>
> >> The internal memslots are much higher than this (0xfffbc000 and
> >> 0xfee00000).  I'm changing the patch to block 0xfe0000000 and above,
> >> otherwise it breaks vmx_dirty_log_test.
> >
> > Perhaps we're working in different units, but I believe paddrs
> > 0xfffbc000 and 0xfee00000 are between 3GiB and 4GiB.
> > "Proof by Python":
>
> I invoke the "not a native speaker" card.  Rephrasing: there is a large
> part at the beginning of the area between 3GiB and 4GiB that isn't used
> by internal memslot (but is used by vmx_dirty_log_test).

Ah, that makes perfect sense, thank you for clarifying. I think the
3G-4G in my head may have come from the x86 PCI hole or similar. In
any case, reducing the prohibited range to just the range covered by
internal memslots feels like a good change.

> Though I have no excuse for the extra zero, the range to block is
> 0xfe000000 to 0x100000000.
>
> Paolo
>
> >>>> B=1
> >>>> KB=1024*B
> >>>> MB=1024*KB
> >>>> GB=1024*MB
> >>>> hex(3*GB)
> > '0xc0000000'
> >>>> hex(4*GB)
> > '0x100000000'
> >>>> 3*GB == 3<<30
> > True
> >>>> 0xfffbc000 > 3*GB
> > True
> >>>> 0xfffbc000 < 4*GB
> > True
> >>>> 0xfee00000 > 3*GB
> > True
> >>>> 0xfee00000 < 4*GB
> > True
> >
> > Am I missing something?
> >
> > I don't think blocking 0xfe0000000 and above is useful, as there's
> > nothing mapped in that region and AFAIK it's perfectly valid to create
> > memslots there.
> >
> >
> >>
> >> Paolo
> >>
> >
>