Re: [PATCH v2 1/6] xen/hvm: Permit HVM_PARAM_IDENT_PT to be set more than once

[Hongyang Yang <yanghy@cn.fujitsu.com>]

I've reviewed all of the series, looks good to me in general, I will rebase my
patches on top of this series. Would you like me to take this series and send
it together with mine?

On 05/09/2015 05:14 AM, Andrew Cooper wrote:
> There is no conceptual problem with setting this parameter more than once.
> Checkpointed migration streams will typically set it once per checkpoint to
> the same value.
>
> The parameter is only actually needed on early-generation VT-x which lacked
> the unrestricted guest capability, although it could plausibly be used on
> newer VT-x with unusual execution control settings.  Short circuit the
> expensive operations on non VT-x hardware.
>
> The parameter itself must always be latched to avoid issues if the VM
> eventually migrates to a host which needs to use the pagetable.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> CC: Keir Fraser <keir@xen.org>
> CC: Jan Beulich <JBeulich@suse.com>
> CC: Yang Hongyang <yanghy@cn.fujitsu.com>
>
> ---
> v2: Don't expose new value prematurely to VT-x
> ---
>   xen/arch/x86/hvm/hvm.c |   14 ++++++++------
>   1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> index 8140a27..85dfa52 100644
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -5730,13 +5730,15 @@ static int hvmop_set_param(
>               rc = -EINVAL;
>           break;
>       case HVM_PARAM_IDENT_PT:
> -        rc = -EINVAL;
> -        if ( d->arch.hvm_domain.params[a.index] != 0 )
> -            break;
> -
> -        rc = 0;
> -        if ( !paging_mode_hap(d) )
> +        /*
> +         * Only actually required for VT-x lacking unrestricted_guest
> +         * capabilities.  Short circuit the pause if possible.
> +         */
> +        if ( !paging_mode_hap(d) || !cpu_has_vmx )
> +        {
> +            d->arch.hvm_domain.params[a.index] = a.value;
>               break;
> +        }
>
>           /*
>            * Update GUEST_CR3 in each VMCS to point at identity map.
>

-- 
Thanks,
Yang.