Re: [Xen-devel] [PATCH RFC v13 06/20] pvh: vmx-specific changes

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.10.13 at 18:12, Tim Deegan <tim@xxxxxxx> wrote:
> At 17:06 +0100 on 07 Oct (1381165615), George Dunlap wrote:
>> On 07/10/13 16:55, Roger Pau Monnà wrote:
>> >On 23/09/13 18:49, George Dunlap wrote:
>> >>@@ -1028,12 +1129,28 @@ static int construct_vmcs(struct vcpu *v)
>> >>                | (1U << TRAP_no_device);
>> >>      vmx_update_exception_bitmap(v);
>> >>  
>> >>+    /* In HVM domains, this happens on the realmode->paging
>> >>+     * transition.  Since PVH never goes through this transition, we
>> >>+     * need to do it at start-of-day. */
>> >>+    if ( is_pvh_domain(d) )
>> >>+        vmx_update_debug_state(v);
>> >>+
>> >>      v->arch.hvm_vcpu.guest_cr[0] = X86_CR0_PE | X86_CR0_ET;
>> >>+
>> >>+    /* PVH domains always start in paging mode */
>> >>+    if ( is_pvh_domain(d) )
>> >>+        v->arch.hvm_vcpu.guest_cr[0] |= X86_CR0_PG | X86_CR0_NE | 
>> >>X86_CR0_WP;
>> >>+
>> >>      hvm_update_guest_cr(v, 0);
>> >>  
>> >>-    v->arch.hvm_vcpu.guest_cr[4] = 0;
>> >>+    v->arch.hvm_vcpu.guest_cr[4] = is_pvh_domain(d) ?
>> >>+        real_cr4_to_pv_guest_cr4(mmu_cr4_features)
>> >Here we need to mask the bits in CR4 that the guest isn't allowed to
>> >set. Right now Xen is setting the VMXE bit by default, which the guest
>> >is not able to modify, so if the guests tries to update CR4 based on the
>> >previous value Xen is going to complain:
>> >
>> >+        real_cr4_to_pv_guest_cr4(mmu_cr4_features) &
>> >+        ~HVM_CR4_GUEST_RESERVED_BITS(v)
>> 
>> Thanks for testing that -- I'll include it in the next spin-up.
> 
> <harping on> This is the sort of thing that would be easier if PVH guests
> were just HVM ones with extra hypercalls. <harping off>

And indeed I'm rather puzzled by the simplistic re-use of a PV
construct: There are memory management related bits in CR4,
and with memory management not being PV for PVH guests,
this seems conceptually wrong.

Jan

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