Re: [PATCH v2] KVM: x86: Fix DR7 mask on task-switch while debugging

[Paolo Bonzini <pbonzini@redhat.com>]

On 19/04/2015 20:12, Nadav Amit wrote:
> If the host sets hardware breakpoints to debug the guest, and a task-switch
> occurs in the guest, the architectural DR7 will not be updated. The effective
> DR7 would be updated instead.
> 
> This fix puts the DR7 update during task-switch emulation, so it now uses the
> standard DR setting mechanism instead of the one that was previously used. As a
> bonus, the update of DR7 will now be effective for AMD as well.
> 
> Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
> 
> ---
> 
> v1 -> v2:
> - Move the setting to emulate.c instead of x86.c
> - Shorten title
> ---
>  arch/x86/kvm/emulate.c | 6 +++++-
>  arch/x86/kvm/vmx.c     | 3 ---
>  2 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 630bcb0..4a4555a 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -25,6 +25,7 @@
>  #include <linux/module.h>
>  #include <asm/kvm_emulate.h>
>  #include <linux/stringify.h>
> +#include <asm/debugreg.h>
>  
>  #include "x86.h"
>  #include "tss.h"
> @@ -2849,7 +2850,7 @@ static int emulator_do_task_switch(struct x86_emulate_ctxt *ctxt,
>  	ulong old_tss_base =
>  		ops->get_cached_segment_base(ctxt, VCPU_SREG_TR);
>  	u32 desc_limit;
> -	ulong desc_addr;
> +	ulong desc_addr, dr7;
>  
>  	/* FIXME: old_tss_base == ~0 ? */
>  
> @@ -2934,6 +2935,9 @@ static int emulator_do_task_switch(struct x86_emulate_ctxt *ctxt,
>  		ret = em_push(ctxt);
>  	}
>  
> +	ops->get_dr(ctxt, 7, &dr7);
> +	ops->set_dr(ctxt, 7, dr7 & ~(DR_LOCAL_ENABLE_MASK | DR_LOCAL_SLOWDOWN));
> +
>  	return ret;
>  }
>  
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index f7a0a7f..8f731c0 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -5703,9 +5703,6 @@ static int handle_task_switch(struct kvm_vcpu *vcpu)
>  		return 0;
>  	}
>  
> -	/* clear all local breakpoint enable flags */
> -	vmcs_writel(GUEST_DR7, vmcs_readl(GUEST_DR7) & ~0x155);
> -
>  	/*
>  	 * TODO: What about debug traps on tss switch?
>  	 *       Are we supposed to inject them and update dr6?
> 

Applied, thanks.

Paolo