Re: [Xen-devel] [PATCH 0/1] ARM: Implement support for write-ctrlreg vm-events

[Corneliu ZUZU <czuzu@xxxxxxxxxxxxxxx>]

On 3/3/2016 4:10 PM, Corneliu ZUZU wrote:
> Then,
> QUESTIONS (FOR VM-EVENTS & ARM MAINTAINERS ESPECIALLY):
>
> Q1) [...]
>
> Q2) [...]
>
> Q3) [...]
>
> Q4) [...]
Hey all,

I have a question relating to this part of code @ vmx_update_guest_cr:

        if ( paging_mode_hap(v->domain) )
        {
            /* Manage GUEST_CR3 when CR0.PE=0. */
            uint32_t cr3_ctls = (CPU_BASED_CR3_LOAD_EXITING |
                                 CPU_BASED_CR3_STORE_EXITING);
            v->arch.hvm_vmx.exec_control &= ~cr3_ctls;
            if ( !hvm_paging_enabled(v) && !vmx_unrestricted_guest(v) )
                v->arch.hvm_vmx.exec_control |= cr3_ctls;

            /* Trap CR3 updates if CR3 memory events are enabled. */
            if ( v->domain->arch.monitor.write_ctrlreg_enabled &
                 monitor_ctrlreg_bitmask(VM_EVENT_X86_CR3) )
                v->arch.hvm_vmx.exec_control |= CPU_BASED_CR3_LOAD_EXITING;

            vmx_update_cpu_exec_control(v);
        }

While trying to move the check for VM_EVENT_X86_CR3 to the scheduling 
tail, a few questions came to my mind.
1). Tamas, Razvan, maybe you guys could clarify this. I noticed this 
part of code is only executed if paging_mode_hap(v->domain). Is EPT 
mandatory to monitor CR3 writes or is it just that when shadow paging is 
enabled, CR3 r/w are unconditionally trapped? If the former is true, 
shouldn't we do a check like this in vm_event_monitor_get_capabilities 
instead?
2). I was also wondering why CR3 load/stores are trapped if paging is 
disabled for a domain.
Thanks,
Corneliu.

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