linux-kernel - Re: [GIT PULL] KVM changes for Linux 5.6-rc4

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Date:   Tue, 03 Mar 2020 14:38:29 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Oliver Upton <oupton@...gle.com>
Subject: Re: [GIT PULL] KVM changes for Linux 5.6-rc4

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 03/03/20 14:02, Vitaly Kuznetsov wrote:
>> Right you are,
>> 
>> a big hammer like
>> 
>> diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
>> index 2a8f2bd..52c9bce 100644
>> --- a/arch/x86/include/asm/kvm_emulate.h
>> +++ b/arch/x86/include/asm/kvm_emulate.h
>> @@ -324,14 +324,6 @@ struct x86_emulate_ctxt {
>>          */
>>  
>>         /* current opcode length in bytes */
>> -       u8 opcode_len;
>> -       u8 b;
>> -       u8 intercept;
>> -       u8 op_bytes;
>> -       u8 ad_bytes;
>> -       struct operand src;
>> -       struct operand src2;
>> -       struct operand dst;
>>         union {
>>                 int (*execute)(struct x86_emulate_ctxt *ctxt);
>>                 fastop_t fop;
>> @@ -343,6 +335,14 @@ struct x86_emulate_ctxt {
>>          * or elsewhere
>>          */
>>         bool rip_relative;
>> +       u8 opcode_len;
>> +       u8 b;
>> +       u8 intercept;
>> +       u8 op_bytes;
>> +       u8 ad_bytes;
>> +       struct operand src;
>> +       struct operand src2;
>> +       struct operand dst;
>>         u8 rex_prefix;
>>         u8 lock_prefix;
>>         u8 rep_prefix;
>> 
>> seems to make the issue go away. (For those wondering why fielf
>> shuffling makes a difference: init_decode_cache() clears
>> [rip_relative, modrm) range) How did this even work before...
>> (I'm still looking at the code, stay tuned...)
>
> On AMD, probably because all these instructions were normally trapped by L1.
>
> Of these, however, most need not be zeroed again. op_bytes, ad_bytes,
> opcode_len and b are initialized by x86_decode_insn, and dst/src/src2
> also by decode_operand.  So only intercept is affected, adding
> "ctxt->intercept = x86_intercept_none" should be enough.

This matches my findings, thank you! Patch[es] are coming.

-- 
Vitaly