From: Nadav Amit <nadav.amit@gmail.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Xu <peterx@redhat.com>, linmiaohe <linmiaohe@huawei.com>,
Sean Christopherson <sean.j.christopherson@intel.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"kvm@vger.kernel.org" <kvm@vger.kernel.org>
Subject: Re: [PATCH] KVM: X86: Avoid explictly fetch instruction in x86_decode_insn()
Date: Wed, 4 Mar 2020 12:30:30 -0800 [thread overview]
Message-ID: <13E8DCB7-D977-49AD-B63F-8BF4B06B096E@gmail.com> (raw)
In-Reply-To: <ad023c34-9a08-7d61-22de-911c4e8760ba@redhat.com>
> On Mar 4, 2020, at 10:19 AM, Paolo Bonzini <pbonzini@redhat.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.
next prev parent reply other threads:[~2020-03-04 20:30 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-04 2:37 [PATCH] KVM: X86: Avoid explictly fetch instruction in x86_decode_insn() linmiaohe
2020-03-04 7:30 ` Paolo Bonzini
2020-03-04 15:18 ` Peter Xu
2020-03-04 15:32 ` Peter Xu
2020-03-04 18:19 ` Paolo Bonzini
2020-03-04 20:30 ` Nadav Amit [this message]
-- strict thread matches above, loose matches on Subject: below --
2020-03-04 2:16 Peter Xu
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