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

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Date:   Sat, 25 Jan 2020 10:37:37 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.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 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).

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
>>
>