Re: [Xen-devel] [PATCH v2 3/4] x86/nvmx: split updating RVI from SVI in nvmx_update_apicv

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> Sent: Wednesday, March 25, 2020 6:19 PM
> 
> Updating SVI is required when an interrupt has been injected using the
> Ack on exit VMEXIT feature, so that the in service interrupt in the
> GUEST_INTR_STATUS matches the vector that is signaled in
> VM_EXIT_INTR_INFO.
> 
> Updating RVI however is not tied to the Ack on exit feature, as it
> signals the next vector to be injected, and hence should always be
> updated to the next pending vector, regardless of whether Ack on exit
> is enabled.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
>  xen/arch/x86/hvm/vmx/vvmx.c | 22 +++++++++++++++++-----
>  1 file changed, 17 insertions(+), 5 deletions(-)
> 
> diff --git a/xen/arch/x86/hvm/vmx/vvmx.c b/xen/arch/x86/hvm/vmx/vvmx.c
> index 1753005c91..8431c912a1 100644
> --- a/xen/arch/x86/hvm/vmx/vvmx.c
> +++ b/xen/arch/x86/hvm/vmx/vvmx.c
> @@ -1384,26 +1384,38 @@ static void nvmx_update_apicv(struct vcpu *v)
>      struct nestedvmx *nvmx = &vcpu_2_nvmx(v);
>      unsigned long reason = get_vvmcs(v, VM_EXIT_REASON);
>      unsigned long intr_info = get_vvmcs(v, VM_EXIT_INTR_INFO);
> +    int rvi;
> 
>      if ( reason == EXIT_REASON_EXTERNAL_INTERRUPT &&
>           nvmx->intr.source == hvm_intsrc_lapic &&
>           (intr_info & INTR_INFO_VALID_MASK) )
>      {
>          uint16_t status;
> -        uint32_t rvi, ppr;
> -        uint32_t vector = intr_info & 0xff;
> +        uint32_t ppr;
> +        unsigned int vector = intr_info & INTR_INFO_VECTOR_MASK;
>          struct vlapic *vlapic = vcpu_vlapic(v);
> 
> +        /*
> +         * Update SVI to record the currently in service interrupt that's
> +         * signaled in EXIT_INTR_INFO.
> +         */
>          vlapic_ack_pending_irq(v, vector, 1);
> 
>          ppr = vlapic_set_ppr(vlapic);
>          WARN_ON((ppr & 0xf0) != (vector & 0xf0));
> 
>          status = vector << VMX_GUEST_INTR_STATUS_SVI_OFFSET;
> -        rvi = vlapic_has_pending_irq(v);
> -        if ( rvi != -1 )
> -            status |= rvi & VMX_GUEST_INTR_STATUS_SUBFIELD_BITMASK;
> +        __vmwrite(GUEST_INTR_STATUS, status);
> +    }
> +
> +    rvi = vlapic_has_pending_irq(v);
> +    if ( rvi != -1 )
> +    {
> +        unsigned long status;
> 
> +        __vmread(GUEST_INTR_STATUS, &status);
> +        status &= ~VMX_GUEST_INTR_STATUS_SUBFIELD_BITMASK;
> +        status |= rvi & VMX_GUEST_INTR_STATUS_SUBFIELD_BITMASK;
>          __vmwrite(GUEST_INTR_STATUS, status);
>      }

I have two minor comments. First, original code requires only one
__vmwrite but now needs two writes and one read for Ack-on-exit
is set. Can we optimize to sustain the original behavior? Second,
although I didn't come up a case where always updating RVI (w/
pending interrupt) may bring some bad effect, it is anyway safer
to put the whole logic within the check of vmexit reason and intr 
source.

Thanks
Kevin