linux-kernel - Re: [RFC PATCH V3 09/11] HV/IOMMU: Enable swiotlb bounce buffer for Isolation VM

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Date:   Thu, 3 Jun 2021 23:37:06 +0800
From:   Tianyu Lan <ltykernel@...il.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>, kys@...rosoft.com,
        haiyangz@...rosoft.com, sthemmin@...rosoft.com, wei.liu@...nel.org,
        decui@...rosoft.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com, arnd@...db.de,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
        rppt@...nel.org, hannes@...xchg.org, cai@....pw,
        krish.sadhukhan@...cle.com, saravanand@...com,
        Tianyu.Lan@...rosoft.com, konrad.wilk@...cle.com, hch@....de,
        m.szyprowski@...sung.com, robin.murphy@....com, jgross@...e.com,
        sstabellini@...nel.org, joro@...tes.org, will@...nel.org,
        xen-devel@...ts.xenproject.org, davem@...emloft.net,
        kuba@...nel.org, jejb@...ux.ibm.com, martin.petersen@...cle.com
Cc:     iommu@...ts.linux-foundation.org, linux-arch@...r.kernel.org,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-scsi@...r.kernel.org, netdev@...r.kernel.org,
        vkuznets@...hat.com, thomas.lendacky@....com,
        brijesh.singh@....com, sunilmut@...rosoft.com
Subject: Re: [RFC PATCH V3 09/11] HV/IOMMU: Enable swiotlb bounce buffer for
 Isolation VM

On 6/3/2021 12:02 AM, Boris Ostrovsky wrote:
> 
> On 6/2/21 11:01 AM, Tianyu Lan wrote:
>> Hi Boris:
>>      Thanks for your review.
>>
>> On 6/2/2021 9:16 AM, Boris Ostrovsky wrote:
>>>
>>> On 5/30/21 11:06 AM, Tianyu Lan wrote:
>>>> @@ -91,6 +92,6 @@ int pci_xen_swiotlb_init_late(void)
>>>>    EXPORT_SYMBOL_GPL(pci_xen_swiotlb_init_late);
>>>>      IOMMU_INIT_FINISH(2,
>>>> -          NULL,
>>>> +          hyperv_swiotlb_detect,
>>>>              pci_xen_swiotlb_init,
>>>>              NULL);
>>>
>>>
>>> Could you explain this change?
>>
>> Hyper-V allocates its own swiotlb bounce buffer and the default
>> swiotlb buffer should not be allocated. swiotlb_init() in pci_swiotlb_init() is to allocate default swiotlb buffer.
>> To achieve this, put hyperv_swiotlb_detect() as the first entry in the iommu_table_entry list. The detect loop in the pci_iommu_alloc() will exit once hyperv_swiotlb_detect() is called in Hyper-V VM and other iommu_table_entry callback will not be called.
> 
> 
> 
> Right. But pci_xen_swiotlb_detect() will only do something for Xen PV guests, and those guests don't run on hyperV. It's either xen_pv_domain() (i.e. hypervisor_is_type(X86_HYPER_XEN_PV)) or hypervisor_is_type(X86_HYPER_MS_HYPERV) but never both. So I don't think there needs to be a dependency between the two callbacks.

Yes, the dependency is between hyperv_swiotlb_detect() and
pci_swiotlb_detect_override()/pci_swiotlb_detect_4gb(). Now
pci_swiotlb_detect_override() and pci_swiotlb_detect_4gb() depends on
pci_xen_swiotlb_detect(). To keep dependency between
hyperv_swiotlb_detect() and pci_swiotlb_detect_override/4gb(), make 
pci_xen_swiotlb_detect() depends on hyperv_swiotlb_detect() and just to
keep order in the IOMMU table. Current iommu_table_entry only has one
depend callback and this is why I put xen depends on hyperv detect function.

Thanks.