Re: [Qemu-devel] [Qemu-block] [PATCH] Added iopmem device emulation

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Tue, Oct 18, 2016 at 03:55:11PM -0600, Logan Gunthorpe wrote:
> An iopmem device is one which exposes volatile or non-volatile memory mapped
> directly to a BAR region. One purpose is to provide buffers to do peer
> to peer transfers on the bus. As such this device uses QEMU's drive
> backing store to simulate non-volatile memory and provides through a
> mapped BAR window.
> 
> This patch creates an emulated device which helps to test and debug the
> kernel driver for iopmem while hardware availability is poor. A kernel
> patch for a driver is being prepared simultaneously.
> 
> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> Signed-off-by: Stephen Bates <sbates@raithlin.com>

QEMU already has NVDIMM support (https://pmem.io/).  It can be used both
for passthrough and fake non-volatile memory:

  qemu-system-x86_64 \
    -M pc,nvdimm=on \
    -m 1024,maxmem=$((4096 * 1024 * 1024)),slots=2 \
    -object memory-backend-file,id=mem0,mem-path=/tmp/foo,size=$((64 * 1024 * 1024)) \
    -device nvdimm,memdev=mem0

Please explain where iopmem comes from, where the hardware spec is, etc?

Perhaps you could use nvdimm instead of adding a new device?

> ---
>  default-configs/pci.mak  |   1 +
>  hw/block/Makefile.objs   |   1 +
>  hw/block/iopmem.c        | 141 +++++++++++++++++++++++++++++++++++++++++++++++
>  include/hw/pci/pci_ids.h |   2 +
>  4 files changed, 145 insertions(+)
>  create mode 100644 hw/block/iopmem.c
> 
> diff --git a/default-configs/pci.mak b/default-configs/pci.mak
> index fff7ce3..aef2b35 100644
> --- a/default-configs/pci.mak
> +++ b/default-configs/pci.mak
> @@ -39,3 +39,4 @@ CONFIG_VGA=y
>  CONFIG_VGA_PCI=y
>  CONFIG_IVSHMEM=$(CONFIG_EVENTFD)
>  CONFIG_ROCKER=y
> +CONFIG_IOPMEM_PCI=y
> diff --git a/hw/block/Makefile.objs b/hw/block/Makefile.objs
> index d4c3ab7..1c8044b 100644
> --- a/hw/block/Makefile.objs
> +++ b/hw/block/Makefile.objs
> @@ -8,6 +8,7 @@ common-obj-$(CONFIG_XEN_BACKEND) += xen_disk.o
>  common-obj-$(CONFIG_ECC) += ecc.o
>  common-obj-$(CONFIG_ONENAND) += onenand.o
>  common-obj-$(CONFIG_NVME_PCI) += nvme.o
> +common-obj-$(CONFIG_IOPMEM_PCI) += iopmem.o
>  
>  obj-$(CONFIG_SH4) += tc58128.o
>  
> diff --git a/hw/block/iopmem.c b/hw/block/iopmem.c
> new file mode 100644
> index 0000000..9c2d716
> --- /dev/null
> +++ b/hw/block/iopmem.c
> @@ -0,0 +1,141 @@
> +/*
> + * QEMU iopmem Controller
> + *
> + * Copyright (c) 2016, Microsemi Corporation
> + *
> + * Written by Logan Gunthorpe <logang@deltatee.com>
> + *
> + * This code is licensed under the GNU GPL v2 or later.
> + */
> +
> +
> +/**
> + * Usage: add options:
> + *      -drive file=<file>,if=none,id=<drive_id>
> + *      -device iopmem,drive=<drive_id>,id=<id[optional]>
> + */
> +
> +#include "qemu/osdep.h"
> +#include "hw/pci/pci.h"
> +#include "sysemu/block-backend.h"
> +
> +typedef struct IoPmemCtrl {
> +    PCIDevice    parent_obj;
> +    MemoryRegion iomem;
> +    BlockBackend *blk;
> +    uint64_t     size;
> +} IoPmemCtrl;
> +
> +#define TYPE_IOPMEM "iopmem"
> +#define IOPMEM(obj) \
> +        OBJECT_CHECK(IoPmemCtrl, (obj), TYPE_IOPMEM)
> +
> +static uint64_t iopmem_bar_read(void *opaque, hwaddr addr, unsigned size)
> +{
> +    IoPmemCtrl *ipm = (IoPmemCtrl *)opaque;
> +    uint64_t val;
> +
> +    blk_pread(ipm->blk, addr, &val, size);
> +
> +    return val;
> +}
> +
> +static void iopmem_bar_write(void *opaque, hwaddr addr, uint64_t data,
> +                           unsigned size)
> +{
> +    IoPmemCtrl *ipm = (IoPmemCtrl *)opaque;
> +
> +    if (addr & 3) {
> +        return;
> +    }
> +
> +    blk_pwrite(ipm->blk, addr, &data, size, 0);
> +}

This has terrible performance because blk_pread()/blk_pwrite() are
synchronous operations.  The vcpu thread is paused while QEMU is waiting
for I/O.

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