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

[Tianyu Lan]

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.