linux-kernel - Re: [PATCH] KVM: X86: Avoid explictly fetch instruction in x86_decode

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Date:   Wed, 4 Mar 2020 12:30:30 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Peter Xu <peterx@...hat.com>, linmiaohe <linmiaohe@...wei.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH] KVM: X86: Avoid explictly fetch instruction in
 x86_decode_insn()

> On Mar 4, 2020, at 10:19 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> 
> On 04/03/20 16:32, Peter Xu wrote:
>>> Looks good, thanks. But it seems we should also take care of the comment in __do_insn_fetch_bytes(), as we do not
>>> load instruction at the beginning of x86_decode_insn() now, which may be misleading:
>>> 		/*
>>>         * One instruction can only straddle two pages,
>>>         * and one has been loaded at the beginning of
>>>         * x86_decode_insn.  So, if not enough bytes
>>>         * still, we must have hit the 15-byte boundary.
>>>         */
>>>        if (unlikely(size < op_size))
>>>                return emulate_gp(ctxt, 0);
>> Right, thanks for spotting that (even if the patch to be dropped :).
>> 
>> I guess not only the comment, but the check might even fail if we
>> apply the patch. Because when the fetch is the 1st attempt and
>> unluckily that acrosses one page boundary (because we'll only fetch
>> until either 15 bytes or the page boundary), so that single fetch
>> could be smaller than op_size provided.
> 
> Right, priming the decode cache with one byte from the current page
> cannot fail, and then we know that the next call must be at the
> beginning of the next page.

IIRC I encountered (and fixed) a similar KVM bug in the past. It is a shame
I never wrote a unit test (and I don’t have time now), but it would be nice
to have one.