Re: [patch] VMX Unrestricted mode support

From: Nitin A Kamble <nitin.a.kamble@intel.com>
To: Avi Kivity <avi@redhat.com>
Cc: "kvm@vger.kernel.org" <kvm@vger.kernel.org>, "; jun.nakajima"@intel.com
Subject: Re: [patch] VMX Unrestricted mode support
Date: Wed, 03 Jun 2009 11:08:55 -0700	[thread overview]
Message-ID: <1244052535.26481.22.camel@mukti.sc.intel.com> (raw)
In-Reply-To: <4A24201A.5060409@redhat.com>

Hi Avi,
  I find that the qemu processor reset state is not per the IA32
processor specifications. (Sections 8.1.1 of
http://www.intel.com/Assets/PDF/manual/253668.pdf)

In qemu-kvm.git in file target-i386/helper.c in function cpu_reset the
segment registers are initialized as follows:

cpu_x86_load_seg_cache(env, R_CS, 0xf000, 0xffff0000, 0xffff,
                           DESC_P_MASK | DESC_S_MASK | DESC_CS_MASK | 
				DESC_R_MASK);
cpu_x86_load_seg_cache(env, R_DS, 0, 0, 0xffff,
                           DESC_P_MASK | DESC_S_MASK | DESC_W_MASK);
cpu_x86_load_seg_cache(env, R_ES, 0, 0, 0xffff,
                           DESC_P_MASK | DESC_S_MASK | DESC_W_MASK);
cpu_x86_load_seg_cache(env, R_SS, 0, 0, 0xffff,
                           DESC_P_MASK | DESC_S_MASK | DESC_W_MASK);
cpu_x86_load_seg_cache(env, R_FS, 0, 0, 0xffff,
                           DESC_P_MASK | DESC_S_MASK | DESC_W_MASK);
cpu_x86_load_seg_cache(env, R_GS, 0, 0, 0xffff,
                           DESC_P_MASK | DESC_S_MASK | DESC_W_MASK);

While the IA32 cpu reset state specification says that Segment Accessed
bit is also 1 at the time of cpu reset. so the above code should look
like this:

cpu_x86_load_seg_cache(env, R_CS, 0xf000, 0xffff0000, 0xffff,
                 DESC_P_MASK | DESC_S_MASK | DESC_CS_MASK | 
                 DESC_R_MASK | DESC_A_MASK);
cpu_x86_load_seg_cache(env, R_DS, 0, 0, 0xffff,
                 DESC_P_MASK | DESC_S_MASK | DESC_W_MASK | DESC_A_MASK);
cpu_x86_load_seg_cache(env, R_ES, 0, 0, 0xffff,
                 DESC_P_MASK | DESC_S_MASK | DESC_W_MASK| DESC_A_MASK);
cpu_x86_load_seg_cache(env, R_SS, 0, 0, 0xffff,
                 DESC_P_MASK | DESC_S_MASK | DESC_W_MASK |DESC_A_MASK);
cpu_x86_load_seg_cache(env, R_FS, 0, 0, 0xffff,
                 DESC_P_MASK | DESC_S_MASK | DESC_W_MASK);
cpu_x86_load_seg_cache(env, R_GS, 0, 0, 0xffff,
                 DESC_P_MASK | DESC_S_MASK | DESC_W_MASK);

This discrepancy is adding the need of the following function in the
unrestricted guest patch.

+static inline u32 get_segment_ar(int seg)
+{
+       if (!enable_unrestricted_guest)
+               return 0xf3;
+
+       switch (seg) {
+       case VCPU_SREG_CS:
+               return 0x9b;
+       case VCPU_SREG_TR:
+               return 0x8b;
+       case VCPU_SREG_LDTR:
+               return 0x82;
+       default:
+               return 0x93;
+       }
+}
+

For the unrestricted guest support either we can fix this discrepancy in
the qemu code, or have a functionality like get_segment_ar() in the kvm
vmx code. 
  what do you suggest ?

Thanks & Regards,
Nitin