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

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Date:   Tue, 03 Mar 2020 14:02:54 +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 02/03/20 19:40, Vitaly Kuznetsov wrote:
>> 
>>  qemu-system-x86-23579 [005] 22018.775584: kvm_exit:             reason EPT_VIOLATION rip 0xfffff802987d6169 info 181 0
>>  qemu-system-x86-23579 [005] 22018.775584: kvm_nested_vmexit:    rip fffff802987d6169 reason EPT_VIOLATION info1 181 info2 0 int_info 0 int_info_err 0
>>  qemu-system-x86-23579 [005] 22018.775585: kvm_page_fault:       address febd0000 error_code 181
>>  qemu-system-x86-23579 [005] 22018.775592: kvm_emulate_insn:     0:fffff802987d6169: f3 a5
>>  qemu-system-x86-23579 [005] 22018.775593: kvm_emulate_insn:     0:fffff802987d6169: f3 a5 FAIL
>>  qemu-system-x86-23579 [005] 22018.775596: kvm_inj_exception:    #UD (0x0)
>> 
>> We probably need to re-enable instruction emulation for something...
>
> This is a rep movsw instruction, it shouldn't be intercepted.  I think
> we have a stale ctxt->intercept because the
>
>         /* Fields above regs are cleared together. */
>
> comment is not true anymore since
>
>     commit c44b4c6ab80eef3a9c52c7b3f0c632942e6489aa
>     Author: Bandan Das <bsd@...hat.com>
>     Date:   Wed Apr 16 12:46:12 2014 -0400
>
>     KVM: emulate: clean up initializations in init_decode_cache
>

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...)

-- 
Vitaly