linux-kernel - Re: [PATCH 1/2] KVM: x86: handle hardware breakpoints during emulation

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Message-ID: <51ADD080.3040308@redhat.com>
Date:	Tue, 04 Jun 2013 13:33:20 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	jan.kiszka@...mens.com
Subject: Re: [PATCH 1/2] KVM: x86: handle hardware breakpoints during emulation

Il 04/06/2013 13:28, Gleb Natapov ha scritto:
> On Thu, May 30, 2013 at 06:00:30PM +0200, Paolo Bonzini wrote:
>> This lets debugging work better during emulation of invalid
>> guest state.
>>
>> The check is done before emulating the instruction, and (in the case
>> of guest debugging) reuses EMULATE_DO_MMIO to exit with KVM_EXIT_DEBUG.
>>
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>> ---
>>  arch/x86/include/asm/kvm_host.h |  3 +-
>>  arch/x86/kvm/x86.c              | 65 +++++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 67 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index e2e09f3..aefd8c2 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -788,9 +788,10 @@ extern u32  kvm_min_guest_tsc_khz;
>>  extern u32  kvm_max_guest_tsc_khz;
>>  
>>  enum emulation_result {
>> -	EMULATE_DONE,       /* no further processing */
>> -	EMULATE_DO_MMIO,      /* kvm_run filled with mmio request */
>> +	EMULATE_DONE,         /* no further processing */
>> +	EMULATE_DO_MMIO,      /* kvm_run ready for userspace exit */
> If it no longer means MMIO (or PIO) lest rename it to something more
> meaningful. EMULATE_EXIT? EMULATE_USER_EXIT?

I'll go with EMULATE_USER_EXIT.

>>  	EMULATE_FAIL,         /* can't emulate this instruction */
>> +	EMULATE_PROCEED,      /* proceed with rest of emulation */
> I think we can do without this. Have to function: check_bp(),
> handle_bp(). Do:
> 
>  if (check_bp())
>    return handle_bp();

I tried this, but it doesn't work because you need to pass the computed
dr6 from check_bp to handle_bp.  It becomes really ugly.

If you do not want EMULATE_PROCEED, I can just use -1 instead in
kvm_vcpu_check_breakpoint, and return if r < 0.

Paolo

>>  };
>>  
>>  #define EMULTYPE_NO_DECODE	    (1 << 0)
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 1d928af..33b51bc 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -4872,6 +4872,60 @@ static bool retry_instruction(struct x86_emulate_ctxt *ctxt,
>>  static int complete_emulated_mmio(struct kvm_vcpu *vcpu);
>>  static int complete_emulated_pio(struct kvm_vcpu *vcpu);
>>  
>> +static int kvm_vcpu_check_hw_bp(unsigned long addr, u32 type, u32 dr7,
>> +				unsigned long *db)
>> +{
>> +	u32 dr6 = 0;
>> +	int i;
>> +	u32 enable, rwlen;
>> +
>> +	enable = dr7;
>> +	rwlen = dr7 >> 16;
>> +	for (i = 0; i < 4; i++, enable >>= 2, rwlen >>= 4)
>> +		if ((enable & 3) && (rwlen & 15) == type && db[i] == addr)
>> +			dr6 |= (1 << i);
>> +	return dr6;
>> +}
>> +
>> +static int kvm_vcpu_check_breakpoint(struct kvm_vcpu *vcpu)
>> +{
>> +	struct kvm_run *kvm_run = vcpu->run;
>> +	unsigned long eip = vcpu->arch.emulate_ctxt.eip;
>> +	u32 dr6 = 0;
>> +
>> +	if (unlikely(vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP) &&
>> +	    (vcpu->arch.guest_debug_dr7 & DR7_BP_EN_MASK)) {
>> +		dr6 = kvm_vcpu_check_hw_bp(eip, 0,
>> +					   vcpu->arch.guest_debug_dr7,
>> +					   vcpu->arch.eff_db);
>> +
>> +		if (dr6 != 0) {
>> +			kvm_run->debug.arch.dr6 = dr6 | DR6_FIXED_1;
>> +			kvm_run->debug.arch.pc = kvm_rip_read(vcpu) +
>> +				get_segment_base(vcpu, VCPU_SREG_CS);
>> +
>> +			kvm_run->debug.arch.exception = DB_VECTOR;
>> +			kvm_run->exit_reason = KVM_EXIT_DEBUG;
>> +			return EMULATE_DO_MMIO;
>> +		}
>> +	}
>> +
>> +	if (unlikely(vcpu->arch.dr7 & DR7_BP_EN_MASK)) {
>> +		dr6 = kvm_vcpu_check_hw_bp(eip, 0,
>> +					   vcpu->arch.dr7,
>> +					   vcpu->arch.db);
>> +
>> +		if (dr6 != 0) {
>> +			vcpu->arch.dr6 &= ~15;
>> +			vcpu->arch.dr6 |= dr6;
>> +			kvm_queue_exception(vcpu, DB_VECTOR);
>> +			return EMULATE_DONE;
>> +		}
>> +	}
>> +
>> +	return EMULATE_PROCEED;
>> +}
>> +
>>  int x86_emulate_instruction(struct kvm_vcpu *vcpu,
>>  			    unsigned long cr2,
>>  			    int emulation_type,
>> @@ -4892,6 +4946,17 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
>>  
>>  	if (!(emulation_type & EMULTYPE_NO_DECODE)) {
>>  		init_emulate_ctxt(vcpu);
>> +
>> +		/*
>> +		 * We will reenter on the same instruction since
>> +		 * we do not set complete_userspace_io.  This does not
>> +		 * handle watchpoints yet, those would be handled in
>> +		 * the emulate_ops.
>> +		 */
>> +		r = kvm_vcpu_check_breakpoint(vcpu);
>> +		if (r != EMULATE_PROCEED)
>> +			return r;
>> +
>>  		ctxt->interruptibility = 0;
>>  		ctxt->have_exception = false;
>>  		ctxt->perm_ok = false;
>> -- 
>> 1.8.1.4
>>
> 
> --
> 			Gleb.
> 

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