Re: [Xen-devel] [PATCH 3/3] x86/vmx: Better description of CR4 settings outside of paged mode

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Sep 29, 2017 at 06:31:03PM +0000, Andrew Cooper wrote:
> This rearanges the logic to avoid the double !hvm_paging_enabled(v) check, but
> is otherwise identical.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
> CC: Kevin Tian <kevin.tian@xxxxxxxxx>
> ---
>  xen/arch/x86/hvm/vmx/vmx.c | 37 ++++++++++++++++++++++++++++---------
>  1 file changed, 28 insertions(+), 9 deletions(-)
> 
> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> index 5b943d4..5b9b074 100644
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -1642,21 +1642,40 @@ static void vmx_update_guest_cr(struct vcpu *v, 
> unsigned int cr)
>          v->arch.hvm_vcpu.hw_cr[4] |= v->arch.hvm_vcpu.guest_cr[4];
>          if ( v->arch.hvm_vmx.vmx_realmode )
>              v->arch.hvm_vcpu.hw_cr[4] |= X86_CR4_VME;
> -        if ( paging_mode_hap(v->domain) && !hvm_paging_enabled(v) )
> -        {
> -            v->arch.hvm_vcpu.hw_cr[4] |= X86_CR4_PSE;
> -            v->arch.hvm_vcpu.hw_cr[4] &= ~X86_CR4_PAE;
> -        }
> +
>          if ( !hvm_paging_enabled(v) )
>          {
>              /*
> -             * SMEP/SMAP is disabled if CPU is in non-paging mode in 
> hardware.
> -             * However Xen always uses paging mode to emulate guest 
> non-paging
> -             * mode. To emulate this behavior, SMEP/SMAP needs to be manually
> -             * disabled when guest VCPU is in non-paging mode.
> +             * When the guest thinks paging is disabled, Xen may need to hide
> +             * the effects of running with CR0.PG actually enabled.  There 
> are
> +             * two subtly complicated cases.
> +             */
> +
> +            if ( paging_mode_hap(v->domain) )
> +            {
> +                /*
> +                 * On hardware lacking the Unrestricted Guest feature (or 
> with
> +                 * it disabled in the VMCS), we may not enter the guest with

Shouldn't this be paging_mode_hap && vmx_unrestricted_guest?

From the code below I think it's harmless what we do with CR4 if
CR0.PG is disabled, but in any case it would be good to mention it in
the comment IMHO.

> +                 * CR0.PG actually disabled.  When EPT is enabled, we run 
> with
> +                 * guest paging settings, but with CR3 pointing at
> +                 * HVM_PARAM_IDENT_PT which is a 32bit pagetable using 4M
> +                 * superpages.  Override the guests paging settings to match.
> +                 */
> +                v->arch.hvm_vcpu.hw_cr[4] |= X86_CR4_PSE;
> +                v->arch.hvm_vcpu.hw_cr[4] &= ~X86_CR4_PAE;

Thanks, Roger.

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