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Re: [Xen-devel] [PATCH v3 2/3] x86/svm: Drop enum instruction_index and simplify svm_get_insn_len()
- To: Jan Beulich <JBeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 31 Jan 2019 18:07:36 +0000
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- Cc: Wei Liu <wei.liu2@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>, Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Brian Woods <brian.woods@xxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Thu, 31 Jan 2019 18:26:54 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 07/01/2019 10:30, Jan Beulich wrote:
>>>> On 31.12.18 at 12:37, <andrew.cooper3@xxxxxxxxxx> wrote:
>> Passing a 32-bit integer index into an array with entries containing less
>> than
>> 32 bits of data is wasteful, and creates an unnecessary error condition of
>> passing an out-of-range index.
>>
>> The width of the X86EMUL_OPC() encoding is at most 24 bits, which leaves room
>> for a modrm byte.
> That's true for the 0x0f-prefix-space insns (and it's just 20 bits in that
> case), but going this route we'd paint ourselves into a corner if we'd
> ever have to add 0x0f38-, 0x0f3a-, or 0x8f0?-prefix-space insns.
We only need constants here for instructions which have intercepts. I
doubt any SIMD instructions are going to enter that category, but we can
always reconsider the encoding scheme (probably to unsigned long) if
this becomes an issue.
> Furthermore someone adjusting the encoding layout in x86_emulate.h
> is very unlikely to notice breakage here until trying the resulting
> binary - I strongly think some BUILD_BUG_ON() should be added to
> make this apparent at build time.
It turns out that BUILD_BUG_ON() doesn't work, because the macro
truncates at 32 bits due to types.
Using
#define INSTR_PAUSE INSTR_ENC(X86EMUL_OPC_F3(0l, 0x90), 0)
i.e. using a long constant for for the ext field does end up triggering
-Woverflow when I artificially set a high bit inside X86EMUL_OPC()
However, this isn't viable protection, as internal changes in
X86EMUL_OPC() would render it useless.
Given that a build time check is proving complicated, and that encoding
errors will be blindingly obvious in debug builds (as the diagnostics
will trigger and we'll hand #GP to the guest), and that it is unlikely
that we're going to vastly change the encoding scheme, I think it will
be fine to go without a build-time check.
~Andrew
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