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    SubjectRe: [PATCH] SCSI driver for VMware's virtual HBA.
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    On Mon, 2009-08-31 at 14:53 -0700, Alok Kataria wrote:
    > > > diff --git a/drivers/scsi/pvscsi.c b/drivers/scsi/pvscsi.c
    > > > new file mode 100644
    > > > index 0000000..fc85a3a
    > > > --- /dev/null
    > > > +++ b/drivers/scsi/pvscsi.c
    > > [...]
    > > > +static int __devinit pvscsi_allocate_sg(struct pvscsi_adapter *adapter)
    > > > +{
    > > > + struct pvscsi_ctx *ctx;
    > > > + int i;
    > > > +
    > > > + ctx = adapter->cmd_map;
    > > > + BUILD_BUG_ON(sizeof(struct pvscsi_sg_list) > PAGE_SIZE);
    > > > +
    > > > + for (i = 0; i < adapter->req_depth; ++i, ++ctx) {
    > > > + ctx->sgl = pci_alloc_consistent(adapter->dev, PAGE_SIZE,
    > > > + &ctx->sglPA);
    > >
    > > Why do you need coherent memory for the sg list? Surely the use pattern
    > > of an SG list is that it follows a predefined ownership model between
    > > the driver and the virtual device, thus not really requiring coherent
    > > memory?
    >
    > The SG list needs to be accessed by the hypervisor too, and we need the
    > physical address to access it from the hypervisor. We do that using the
    > sglPA field right now, which the pci_.. interface returns. Do you think
    > something else should be used for that purpose ?

    The hypervisor is part of the OS coherency domain ... coherent memory is
    designed to be a mailbox access between a device and driver, which I
    believe the hypervisor doesn't need.

    If all you're looking for is a way to map a page of memory to a physical
    address, won't dma_map_single() do that for you? Or, actually, in this
    case virt_to_page() might be a better way.

    James




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