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

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Date:   Wed, 2 Jun 2021 12:02:03 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Tianyu Lan <ltykernel@...il.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 6/2/21 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.



-boris