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Re: [Xen-devel] [PATCH] reenabling ptrace for paravirtualized guests

To: Simon Kagstrom <simon.kagstrom@xxxxxx>
Subject: Re: [Xen-devel] [PATCH] reenabling ptrace for paravirtualized guests
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Fri, 12 May 2006 08:19:36 +0100
Cc: xen-devel list <xen-devel@xxxxxxxxxxxxxxxxxxx>, Hollis Blanchard <hollisb@xxxxxxxxxx>, Ryan Harper <ryanh@xxxxxxxxxx>
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On 12 May 2006, at 08:07, Simon Kagstrom wrote:

        ctxt.gs_base_kernel = (unsigned long)(cpu_pda(vcpu));
 #endif
+   /* set sane cr0 bits, protected and paging enabled  */
+   ctxt.ctrlreg[0] = 0x80000001;

        BUG_ON(HYPERVISOR_vcpu_op(VCPUOP_initialise, vcpu, &ctxt));
 }

I'll add this to the patch.

This can be done entirely in Xen without guest modification and avoiding magic numbers.

Add the following to arch_set_info_guest, immediately after the memcpy to guest_context:
  /* Only CR0.TS is modifiable by guest or admin. */
  v->arch.guest_context.ctrl_reg[0] &= X86_CR0_TS;
  v->arch.guest_context.ctrl_reg[0] |= read_cr0() & ~X86_CR0_TS;

Whether we should fill in from real CR0 or simply set a static selection of flags is open to debate. I don't really think it matters either way.

 -- Keir


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