Re: [Xen-devel] [PATCH] Avoid needless flush cache when guest change MTRR

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

> From: Li, Liang Z
> Sent: Wednesday, February 04, 2015 11:28 AM
> 
> Flushing cache is needed only when guest has IOMMU device, using
> need_iommu(d) can minimize the impact to guest with device assigned,
> since a guest may be hot plugged with a device thus there may be dirty
> cache lines before need_iommu(d) becoming true, force the flush_all
> when the first device is assigned to guest to amend this issue.
> 
> Signed-off-by: Liang Li <liang.z.li@xxxxxxxxx>
> Signed-off-by: Yang Zhang <yang.z.zhang@xxxxxxxxx>
> ---
>  xen/arch/x86/hvm/mtrr.c       | 2 +-
>  xen/drivers/passthrough/pci.c | 5 +++++
>  2 files changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/x86/hvm/mtrr.c b/xen/arch/x86/hvm/mtrr.c
> index ee18553..858fb7e 100644
> --- a/xen/arch/x86/hvm/mtrr.c
> +++ b/xen/arch/x86/hvm/mtrr.c
> @@ -791,7 +791,7 @@ HVM_REGISTER_SAVE_RESTORE(MTRR,
> hvm_save_mtrr_msr, hvm_load_mtrr_msr,
> 
>  void memory_type_changed(struct domain *d)
>  {
> -    if ( iommu_enabled && d->vcpu && d->vcpu[0] )
> +    if ( need_iommu(d) && d->vcpu && d->vcpu[0] )
>      {
>          p2m_memory_type_changed(d);
>          flush_all(FLUSH_CACHE);
> diff --git a/xen/drivers/passthrough/pci.c b/xen/drivers/passthrough/pci.c
> index 78c6977..a023d1d 100644
> --- a/xen/drivers/passthrough/pci.c
> +++ b/xen/drivers/passthrough/pci.c
> @@ -1365,6 +1365,11 @@ static int assign_device(struct domain *d, u16 seg,
> u8 bus, u8 devfn)
>              }
>          }
>          d->need_iommu = 1;
> +        /* There may be dirty cache lines when a device is assigned
> +         * and before need_iommu(d) becoming true, this will cause
> +         * memory_type_changed lose effect if memory type changes.
> +         * Call memory_type_changed here to amend this. */
> +        memory_type_changed(d);

could we relax this force flush only on the 1st assigned device which actually
changes need_iommu(d) from false to true?

>      }
> 
>      pdev = pci_get_pdev_by_domain(hardware_domain, seg, bus, devfn);
> --
> 1.9.1


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