Re: [PATCH v5] drivers/misc: vm_gen_counter: initial driver implementation

Michael S. Tsirkin
2018-03-15T13:31:05Z

On Thu, Mar 15, 2018 at 10:00:30AM +0200, Gal Hammer wrote:
> On Tue, Mar 13, 2018 at 7:40 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > Thanks for the patch! Yet something to improve (see below):
>
> Thanks for the review.
>
> > On Thu, Mar 01, 2018 at 04:22:15PM +0200, Or Idgar wrote:
> >> From: Or Idgar <oridgar@gmail.com>
> >
> > I see addresses at gmail, virtualoco and redhat.com At this point I
> > don't really know which address to use to contact you :) All of them?
>
> Use his gmail or virtualoco one.
>
> > Also, I think it's a good idea to CC this more widely. Consider CC-ing
> > qemu contributors to the vm gen id (use git log to get the list), Ben
> > Warren who wrote a prototype driver a while ago, qemu mailing list,
> > maybe more.
> >
> >>
> >> This patch is a driver which expose the Virtual Machine Generation ID
> >> via sysfs.
> >>
> >> The ID is a UUID value used to differentiate between virtual
> >> machines.
> >>
> >> The VM-Generation ID is a feature defined by Microsoft (paper:
> >> http://go.microsoft.com/fwlink/?LinkId=260709) and supported by multiple
> >> hypervisor vendors.
> >>
> >> Signed-off-by: Or Idgar <oridgar@gmail.com>
> >
> > I think it's a good idea to use sysfs for this. However,
> > there are a couple of missing interfaces here:
> >
> > 1. Userspace needs a way to know when this value changes.
> > I see no change notifications here and that does not seem right.
>
> We have the next version which includes notification. The problem is
> that right now QEMU doesn't include a mean to change the generation id
> on-the-fly, so it was not published it.

It seems to send this notification on each migration or
resume from snapshot.

> > system calls. Pls add mmap support to the raw format.
> > (Phys address is not guaranteed to be page-aligned so you will
> > probably want an offset attribute for that as well).
>
> I don't agree that this should be a requirement. According to the
> specs, the value doesn't change frequently.

What matters is whether it's read frequently, not whether it
changes frequently.

Still, I agree we can defer that
for now until apps actually start using the new interface.

> A notification about a
> change the re-reading the value should be enough for now.

Notifications are asynchronous so I am not sure it's robust to rely on
them. So I suspect we'll need it down the road but I agree to defer that
for now until apps actually start using the new interface.

> > While it's possible to add these later in theory, it's
> > easier if userspace can rely on all interfaces being
> > in place just by detecting the directory presence.
> >
> >> ---
> >>
> >> Changes in v5:
> >> - added to VMGENID module dependency on ACPI module.
> >>
> >> Documentation/ABI/testing/sysfs-hypervisor | 13 +++
> >> drivers/misc/Kconfig | 7 ++
> >> drivers/misc/Makefile | 1 +
> >> drivers/misc/vmgenid.c | 142 +++++++++++++++++++++++++++++
> >
> > Do you want to add this under QEMU MACHINE EMULATOR in MAINTAINERS?
> > This way e.g. qemu-devel will be copied.
>
> This feature is supported by other hypervisors and this driver should
> be able to support them in the next versions. I don't see a reason to
> bound it to QEMU.

Go ahead and add the microsoft list too, whatever it is.
Point is you want people familiar with the spec to
look at patches.

> >> 4 files changed, 163 insertions(+)
> >> create mode 100644 Documentation/ABI/testing/sysfs-hypervisor
> >> create mode 100644 drivers/misc/vmgenid.c
> >>
> >> diff --git a/Documentation/ABI/testing/sysfs-hypervisor b/Documentation/ABI/testing/sysfs-hypervisor
> >> new file mode 100644
> >> index 000000000000..2f9a7b8eab70
> >> --- /dev/null
> >> +++ b/Documentation/ABI/testing/sysfs-hypervisor
> >> @@ -0,0 +1,13 @@
> >> +What: /sys/hypervisor/vm_gen_counter
> >
> > It's not a counter, is it?
> > I'd go with "vm_generation_id" here. Eschew abbreviation.
>
> The names were chosen so they'll match Microsoft's specification and
> code examples.

I don't think this makes sense. We are not going to use '\' in file
names to match Microsoft code examples, either.

Maybe they say counter in some docs because someone somewhere uses this
as a counter. Or maybe some architect wrote the code examples before the
name was changed from gen counter to generation id and did not bother
changing the code examples. Who knows - there is still no point in
building an API around that.

In particular QEMU certainly does not pass a counter in that field,
and spec does not say it should.

> >> +Date: February 2018
> >> +Contact: Or Idgar <idgar@virtualoco.com>
> >> + Gal Hammer <ghammer@redhat.com>
> >> +Description: Expose the virtual machine generation ID. The directory
> >> + contains two files: "generation_id" and "raw". Both files
> >> + represent the same information.
> >> +
> >> + "generation_id" file is a UUID string
> >
> > And this I'd call "uuid" then.
>
> Why? The name says what the value is, not its type. This is not common
> to see values' types in the sysfs directory.

Look at the hierarchy:

/sys/hypervisor/vm_gen_counter/generation_id
/sys/hypervisor/vm_gen_counter/raw

Naming it vm_gen_counter/generation_id makes it seem like
there is a gen counter (general counter?) which
allows some kind of raw access and which also has
a generation id.

But in reality what is going on here is that there is a single value
which is a vm generation id, and we present it in two formats: uuid and raw.

> >> + representation.
> >> +
> >> + "raw" file is a 128-bit integer
> >> + representation (binary).
> >> diff --git a/drivers/misc/Kconfig b/drivers/misc/Kconfig
> >> index 03605f8fc0dc..a39feff6a867 100644
> >> --- a/drivers/misc/Kconfig
> >> +++ b/drivers/misc/Kconfig
> >> @@ -500,6 +500,13 @@ config MISC_RTSX
> >> tristate
> >> default MISC_RTSX_PCI || MISC_RTSX_USB
> >>
> >> +config VMGENID
> >> + depends on ACPI
> >> + tristate "Virtual Machine Generation ID driver"
> >> + help
> >> + This is a Virtual Machine Generation ID driver which provides
> >> + a virtual machine unique identifier.
> >> +
> >> source "drivers/misc/c2port/Kconfig"
> >> source "drivers/misc/eeprom/Kconfig"
> >> source "drivers/misc/cb710/Kconfig"

BTW maybe we want to make this depend on CONFIG_PARAVIRT as well.

> >> diff --git a/drivers/misc/Makefile b/drivers/misc/Makefile
> >> index c3c8624f4d95..067aa666bb6a 100644
> >> --- a/drivers/misc/Makefile
> >> +++ b/drivers/misc/Makefile
> >> @@ -57,6 +57,7 @@ obj-$(CONFIG_ASPEED_LPC_SNOOP) += aspeed-lpc-snoop.o
> >> obj-$(CONFIG_PCI_ENDPOINT_TEST) += pci_endpoint_test.o
> >> obj-$(CONFIG_OCXL) += ocxl/
> >> obj-$(CONFIG_MISC_RTSX) += cardreader/
> >> +obj-$(CONFIG_VMGENID) += vmgenid.o
> >>
> >> lkdtm-$(CONFIG_LKDTM) += lkdtm_core.o
> >> lkdtm-$(CONFIG_LKDTM) += lkdtm_bugs.o
> >> diff --git a/drivers/misc/vmgenid.c b/drivers/misc/vmgenid.c
> >> new file mode 100644
> >> index 000000000000..6c8d8fe75335
> >> --- /dev/null
> >> +++ b/drivers/misc/vmgenid.c
> >> @@ -0,0 +1,142 @@
> >> +// SPDX-License-Identifier: GPL-2.0
> >> +/*
> >> + * Virtual Machine Generation ID driver
> >> + *
> >> + * Copyright (C) 2018 Red Hat, Inc. All rights reserved.
> >> + * Authors:
> >> + * Or Idgar <oridgar@gmail.com>
> >> + * Gal Hammer <ghammer@redhat.com>
> >> + *
> >> + */
> >> +#include <linux/init.h>
> >> +#include <linux/module.h>
> >> +#include <linux/kernel.h>
> >> +#include <linux/kobject.h>
> >> +#include <linux/acpi.h>
> >> +#include <linux/uuid.h>
> >> +
> >> +MODULE_LICENSE("GPL");
> >> +MODULE_AUTHOR("Or Idgar <oridgar@gmail.com>");
> >> +MODULE_AUTHOR("Gal Hammer <ghammer@redhat.com>");
> >> +MODULE_DESCRIPTION("Virtual Machine Generation ID");
> >> +MODULE_VERSION("0.1");
> >> +
> >> +ACPI_MODULE_NAME("vmgenid");
> >> +
> >> +static u64 phys_addr;
> >> +
> >> +static ssize_t generation_id_show(struct device *_d,
> >> + struct device_attribute *attr, char *buf)
> >> +{
> >> + void __iomem *uuid_map;
> >> + uuid_t uuid;
> >> + ssize_t result;
> >> +
> >> + uuid_map = acpi_os_map_iomem(phys_addr, sizeof(uuid_t));
> >> + if (!uuid_map)
> >> + return -EFAULT;
> >
> > Shouldn't this be acpi_os_map_memory? Spec says it's never an IO address.
> >
> >> +
> >> + memcpy_fromio(&uuid, uuid_map, sizeof(uuid_t));
> >> + result = sprintf(buf, "%pUl\n", &uuid);
> >> + acpi_os_unmap_iomem(uuid_map, sizeof(uuid_t));
> >> + return result;
> >> +}
> >> +static DEVICE_ATTR_RO(generation_id);
> >> +
> >> +static ssize_t raw_show(struct device *_d,
> >> + struct device_attribute *attr,
> >> + char *buf)
> >> +{
> >> + void __iomem *uuid_map;
> >> +
> >> + uuid_map = acpi_os_map_iomem(phys_addr, sizeof(uuid_t));
> >> + if (!uuid_map)
> >> + return -EFAULT;
> >> + memcpy_fromio(buf, uuid_map, sizeof(uuid_t));
> >> + acpi_os_unmap_iomem(uuid_map, sizeof(uuid_t));
> >> + return sizeof(uuid_t);
> >> +}
> >> +static DEVICE_ATTR_RO(raw);
> >
> > I think you want BIN_ATTR_RO.
> >
> >
> >> +
> >> +static struct attribute *vmgenid_attrs[] = {
> >> + &dev_attr_generation_id.attr,
> >> + &dev_attr_raw.attr,
> >> + NULL,
> >> +};
> >> +static const struct attribute_group vmgenid_group = {
> >> + .name = "vm_gen_counter",
> >> + .attrs = vmgenid_attrs,
> >> +};
> >> +
> >> +static int get_vmgenid(acpi_handle handle)
> >> +{
> >> + int i;
> >> + struct acpi_buffer buffer = { ACPI_ALLOCATE_BUFFER, NULL };
> >> + acpi_status status;
> >> + union acpi_object *pss;
> >> + union acpi_object *element;
> >> +
> >> + status = acpi_evaluate_object(handle, "ADDR", NULL, &buffer);
> >> + if (ACPI_FAILURE(status)) {
> >> + ACPI_EXCEPTION((AE_INFO, status, "Evaluating _ADDR"));
> >
> > It's ADDR - not _ADDR, right?
> >
> >> + return -ENODEV;
> >> + }
> >> + pss = buffer.pointer;
> >> + if (!pss || pss->type != ACPI_TYPE_PACKAGE || pss->package.count != 2)
> >> + return -EFAULT;
> >> +
> >> + phys_addr = 0;
> >> + for (i = 0; i < pss->package.count; i++) {
> >> + element = &(pss->package.elements[i]);
> >> + if (element->type != ACPI_TYPE_INTEGER)
> >> + return -EFAULT;
> >
> > EINVAL here and elsewhere.
> >
> >> + phys_addr |= element->integer.value << i*32;
> >
> > i * 32
> >
> >> + }
> >> + return 0;
> >> +}
> >> +
> >> +static int acpi_vmgenid_add(struct acpi_device *device)
> >> +{
> >> + int retval;
> >> +
> >> + if (!device)
> >> + return -EINVAL;
> >> + retval = get_vmgenid(device->handle);
> >> + if (retval < 0)
> >> + return retval;
> >> + return sysfs_create_group(hypervisor_kobj, &vmgenid_group);
> >> +}
> >> +
> >> +static int acpi_vmgenid_remove(struct acpi_device *device)
> >> +{
> >> + sysfs_remove_group(hypervisor_kobj, &vmgenid_group);
> >> + return 0;
> >> +}
> >> +
> >> +static const struct acpi_device_id vmgenid_ids[] = {
> >> + {"QEMUVGID", 0},
> >> + {"", 0},
> >> +};
> >
> > That's not right I think. You should go by _CID and maybe
> > _DDN.
>
> You are probably right on this. However, we didn't see that Linux
> supports loading ACPI modules other than using the _HID value.

Are you sure about this?
if (info->valid & ACPI_VALID_CID) {
cid_list = &info->compatible_id_list;
for (i = 0; i < cid_list->count; i++)
acpi_add_id(pnp, cid_list->ids[i].string);
}

Anyway, spec is pretty clear on this so we have to make it work.

> >>
> >> +
> >> +static struct acpi_driver acpi_vmgenid_driver = {
> >> + .name = "vm_gen_counter",
> >> + .ids = vmgenid_ids,
> >> + .owner = THIS_MODULE,
> >> + .ops = {
> >> + .add = acpi_vmgenid_add,
> >> + .remove = acpi_vmgenid_remove,
> >> + }
> >> +};
> >> +
> >> +static int __init vmgenid_init(void)
> >> +{
> >> + return acpi_bus_register_driver(&acpi_vmgenid_driver);
> >> +}
> >> +
> >> +static void __exit vmgenid_exit(void)
> >> +{
> >> + acpi_bus_unregister_driver(&acpi_vmgenid_driver);
> >> +}
> >> +
> >> +module_init(vmgenid_init);
> >> +module_exit(vmgenid_exit);
> >
> > Thanks for your work, and I hope this helps!
> >
> > --
> > MST
>
> Thanks,
>
> Gal.