Re: [patch] VMX Unrestricted mode support

[Avi Kivity <avi@xxxxxxxxxx>]

Nitin A Kamble wrote:
> 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 ?
>
>   

Qemu should be fixed of course, but we want kvm to keep working with 
older qemu.  So please also have KVM_SET_SREGS set the A bit on segments 
which are not unusable.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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