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

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Date:   Tue, 3 Mar 2020 14:07:00 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...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

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.

Paolo