linux-kernel - Re: [PATCH V3 11/13] HV/IOMMU: Enable swiotlb bounce buffer for Isolation VM

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Message-ID: <e933f7e3-5573-bb8a-c313-75884d59235c@gmail.com>
Date:   Fri, 20 Aug 2021 17:32:19 +0800
From:   Tianyu Lan <ltykernel@...il.com>
To:     Michael Kelley <mikelley@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
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Cc:     "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
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Subject: Re: [PATCH V3 11/13] HV/IOMMU: Enable swiotlb bounce buffer for
 Isolation VM



On 8/20/2021 2:11 AM, Michael Kelley wrote:
>>   }
>> +
>> +/*
>> + * hv_map_memory - map memory to extra space in the AMD SEV-SNP Isolation VM.
>> + */
>> +void *hv_map_memory(void *addr, unsigned long size)
>> +{
>> +	unsigned long *pfns = kcalloc(size / HV_HYP_PAGE_SIZE,
>> +				      sizeof(unsigned long), GFP_KERNEL);
>> +	void *vaddr;
>> +	int i;
>> +
>> +	if (!pfns)
>> +		return NULL;
>> +
>> +	for (i = 0; i < size / HV_HYP_PAGE_SIZE; i++)
>> +		pfns[i] = virt_to_hvpfn(addr + i * HV_HYP_PAGE_SIZE) +
>> +			(ms_hyperv.shared_gpa_boundary >> HV_HYP_PAGE_SHIFT);
>> +
>> +	vaddr = vmap_pfn(pfns, size / HV_HYP_PAGE_SIZE,	PAGE_KERNEL_IO);
>> +	kfree(pfns);
>> +
>> +	return vaddr;
>> +}
> This function is manipulating page tables in the guest VM.  It is not involved
> in communicating with Hyper-V, or passing PFNs to Hyper-V.  The pfn array
> contains guest PFNs, not Hyper-V PFNs.  So it should use PAGE_SIZE
> instead of HV_HYP_PAGE_SIZE, and similarly PAGE_SHIFT and virt_to_pfn().
> If this code were ever to run on ARM64 in the future with PAGE_SIZE other
> than 4 Kbytes, the use of PAGE_SIZE is correct choice.

OK. Will update with PAGE_SIZE.


> 
>> +void __init hyperv_iommu_swiotlb_init(void)
>> +{
>> +	unsigned long bytes;
>> +
>> +	/*
>> +	 * Allocate Hyper-V swiotlb bounce buffer at early place
>> +	 * to reserve large contiguous memory.
>> +	 */
>> +	hyperv_io_tlb_size = 256 * 1024 * 1024;
> A hard coded size here seems problematic.   The memory size of
> Isolated VMs can vary by orders of magnitude.  I see that
> xen_swiotlb_init() uses swiotlb_size_or_default(), which at least
> pays attention to the value specified on the kernel boot line.
> 
> Another example is sev_setup_arch(), which in the native case sets
> the size to 6% of main memory, with a max of 1 Gbyte.  This is
> the case that's closer to Isolated VMs, so doing something
> similar could be a good approach.
> 

Yes, agree. It's better to keep bounce buffer size with AMD SEV.