linux-kernel - Re: [PATCH 4/7] KVM: vmx: Allow the guest to run with dirty debug registers

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Date:	Sun, 09 Mar 2014 21:12:05 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
CC:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	alex.williamson@...hat.com, mtosatti@...hat.com, gleb@...nel.org,
	jan.kiszka@...mens.com
Subject: Re: [PATCH 4/7] KVM: vmx: Allow the guest to run with dirty debug
 registers

Il 09/03/2014 19:26, Radim Krčmář ha scritto:
> > +
> > +		cpu_based_vm_exec_control = vmcs_read32(CPU_BASED_VM_EXEC_CONTROL);
> > +		cpu_based_vm_exec_control &= ~CPU_BASED_MOV_DR_EXITING;
> > +		vmcs_write32(CPU_BASED_VM_EXEC_CONTROL, cpu_based_vm_exec_control);
>
> vmcs_clear_bits() covers exactly this use-case.
> (Barring the explicit bit-width.)

Good idea.

> > +
> > +		/*
> > +		 * No more DR vmexits; force a reload of the debug registers
> > +		 * and reenter on this instruction.  The next vmexit will
> > +		 * retrieve the full state of the debug registers.
> > +		 */
> > +		vcpu->arch.switch_db_regs |= KVM_DEBUGREG_WONT_EXIT;
> > +		return 1;
> > +	}
> > +
>
> We could make the code slighly uglier and move the functional part of
> this block before the previous one, so it would do both things in one
> exit.

I considered this, but decided that it's unlikely for emulation to be 
faster than hardware---especially on those AMD CPUs that lack decode 
assists (and it's good for VMX and SVM code to look as similar as possible).

> (Exception handler will likely access DR too.)

Which exception handler?

Paolo
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