Re: [RFC PATCH 2/6] virtio: add option to restrict memory access under Xen

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Sun, 17 Apr 2022, Oleksandr wrote:
> On 16.04.22 01:01, Stefano Stabellini wrote:
> > On Thu, 14 Apr 2022, Oleksandr Tyshchenko wrote:
> > > From: Juergen Gross <jgross@xxxxxxxx>
> > > 
> > > In order to support virtio in Xen guests add a config option enabling
> > > the user to specify whether in all Xen guests virtio should be able to
> > > access memory via Xen grant mappings only on the host side.
> > > 
> > > This applies to fully virtualized guests only, as for paravirtualized
> > > guests this is mandatory.
> > > 
> > > This requires to switch arch_has_restricted_virtio_memory_access()
> > > from a pure stub to a real function on x86 systems (Arm systems are
> > > not covered by now).
> > > 
> > > Add the needed functionality by providing a special set of DMA ops
> > > handling the needed grant operations for the I/O pages.
> > > 
> > > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > > ---
> > >   arch/x86/mm/init.c        |  15 ++++
> > >   arch/x86/mm/mem_encrypt.c |   5 --
> > >   arch/x86/xen/Kconfig      |   9 +++
> > >   drivers/xen/Kconfig       |  20 ++++++
> > >   drivers/xen/Makefile      |   1 +
> > >   drivers/xen/xen-virtio.c  | 177
> > > ++++++++++++++++++++++++++++++++++++++++++++++
> > >   include/xen/xen-ops.h     |   8 +++
> > >   7 files changed, 230 insertions(+), 5 deletions(-)
> > >   create mode 100644 drivers/xen/xen-virtio.c
> > > 
> > > diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> > > index d8cfce2..526a3b2 100644
> > > --- a/arch/x86/mm/init.c
> > > +++ b/arch/x86/mm/init.c
> > > @@ -8,6 +8,8 @@
> > >   #include <linux/kmemleak.h>
> > >   #include <linux/sched/task.h>
> > >   +#include <xen/xen.h>
> > > +
> > >   #include <asm/set_memory.h>
> > >   #include <asm/e820/api.h>
> > >   #include <asm/init.h>
> > > @@ -1065,3 +1067,16 @@ unsigned long max_swapfile_size(void)
> > >           return pages;
> > >   }
> > >   #endif
> > > +
> > > +#ifdef CONFIG_ARCH_HAS_RESTRICTED_VIRTIO_MEMORY_ACCESS
> > > +int arch_has_restricted_virtio_memory_access(void)
> > > +{
> > > + if (IS_ENABLED(CONFIG_XEN_PV_VIRTIO) && xen_pv_domain())
> > > +         return 1;
> > > + if (IS_ENABLED(CONFIG_XEN_HVM_VIRTIO_GRANT) && xen_hvm_domain())
> > > +         return 1;
> > I think these two checks could be moved to a separate function in a Xen
> > header, e.g. xen_restricted_virtio_memory_access, and here you could
> > just
> > 
> > if (xen_restricted_virtio_memory_access())
> >      return 1;
> 
> Agree, will do
> 
> 
> > 
> > 
> > 
> > > + return cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT);
> > > +}
> > > +EXPORT_SYMBOL_GPL(arch_has_restricted_virtio_memory_access);
> > > +#endif
> > > diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
> > > index 50d2099..dda020f 100644
> > > --- a/arch/x86/mm/mem_encrypt.c
> > > +++ b/arch/x86/mm/mem_encrypt.c
> > > @@ -77,8 +77,3 @@ void __init mem_encrypt_init(void)
> > >           print_mem_encrypt_feature_info();
> > >   }
> > >   -int arch_has_restricted_virtio_memory_access(void)
> > > -{
> > > - return cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT);
> > > -}
> > > -EXPORT_SYMBOL_GPL(arch_has_restricted_virtio_memory_access);
> > > diff --git a/arch/x86/xen/Kconfig b/arch/x86/xen/Kconfig
> > > index 85246dd..dffdffd 100644
> > > --- a/arch/x86/xen/Kconfig
> > > +++ b/arch/x86/xen/Kconfig
> > > @@ -92,3 +92,12 @@ config XEN_DOM0
> > >           select X86_X2APIC if XEN_PVH && X86_64
> > >           help
> > >             Support running as a Xen Dom0 guest.
> > > +
> > > +config XEN_PV_VIRTIO
> > > + bool "Xen virtio support for PV guests"
> > > + depends on XEN_VIRTIO && XEN_PV
> > > + default y
> > > + help
> > > +   Support virtio for running as a paravirtualized guest. This will
> > > +   need support on the backend side (qemu or kernel, depending on the
> > > +   virtio device types used).
> > > diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> > > index 120d32f..fc61f7a 100644
> > > --- a/drivers/xen/Kconfig
> > > +++ b/drivers/xen/Kconfig
> > > @@ -335,4 +335,24 @@ config XEN_UNPOPULATED_ALLOC
> > >             having to balloon out RAM regions in order to obtain physical 
> > > memory
> > >             space to create such mappings.
> > >   +config XEN_VIRTIO
> > > + bool "Xen virtio support"
> > > + default n
> > > + depends on VIRTIO && DMA_OPS
> > > + select ARCH_HAS_RESTRICTED_VIRTIO_MEMORY_ACCESS
> > > + help
> > > +   Enable virtio support for running as Xen guest. Depending on the
> > > +   guest type this will require special support on the backend side
> > > +   (qemu or kernel, depending on the virtio device types used).
> > > +
> > > +config XEN_HVM_VIRTIO_GRANT
> > > + bool "Require virtio for fully virtualized guests to use grant
> > > mappings"
> > > + depends on XEN_VIRTIO && X86_64
> > > + default y
> > > + help
> > > +   Require virtio for fully virtualized guests to use grant mappings.
> > > +   This will avoid the need to give the backend the right to map all
> > > +   of the guest memory. This will need support on the backend side
> > > +   (qemu or kernel, depending on the virtio device types used).
> > I don't think we need 3 visible kconfig options for this.
> > 
> > In fact, I would only add one: XEN_VIRTIO. We can have any X86 (or ARM)
> > specific dependencies in the "depends" line under XEN_VIRTIO. And I
> > don't think we need XEN_HVM_VIRTIO_GRANT as a kconfig option
> > necessarely. It doesn't seem like some we want as build time option. At
> > most, it could be a runtime option (like a command line) or a debug
> > option (like an #define at the top of the source file.)
> 
> 
> I don't know what was the initial idea of having and extra XEN_HVM_VIRTIO and
> XEN_PV_VIRTIO options, but taking into the account that
> they are only used in arch_has_restricted_virtio_memory_access() currently, I
> share your opinion regarding a single XEN_VIRTIO option.
> 
> Looking ahead (including changes in the commit #4), we can imagine the
> resulting option:
> 
> config XEN_VIRTIO
>     bool "Xen virtio support"
>     default n
>     depends on VIRTIO && DMA_OPS
>     depends on (X86_64 || ARM || ARM64)
>     select ARCH_HAS_RESTRICTED_VIRTIO_MEMORY_ACCESS
>     help
>       Enable virtio support for running as Xen guest. Depending on the
>       guest type this will require special support on the backend side
>       (qemu or kernel, depending on the virtio device types used).
> 
> 
> and then arch_has_restricted_virtio_memory_access() per arch:
> 
> 
> 1. x86:
> 
> int arch_has_restricted_virtio_memory_access(void)
> {
>     return (xen_has_restricted_virtio_memory_access() ||
>             cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT));
> }
> 
> 
> 2. Arm:
> 
> int arch_has_restricted_virtio_memory_access(void)
> {
>     return xen_has_restricted_virtio_memory_access();
> }
> 
> 
> 3. xen.h:
> 
> static inline int xen_has_restricted_virtio_memory_access(void)
> {
>     if (IS_ENABLED(CONFIG_XEN_VIRTIO) && (xen_pv_domain() ||
> xen_hvm_domain()))
>         return 1;
> 
>     return 0;
> }
> 
> 
> Actually, as domain type on Arm is always XEN_HVM_DOMAIN, we could probably
> have the following on Arm:
> 
> int arch_has_restricted_virtio_memory_access(void)
> {
>     return IS_ENABLED(CONFIG_XEN_VIRTIO);
> }
> 
> but I would prefer not to diverge and use common
> xen_has_restricted_virtio_memory_access().
> 
> Any thoughts?

Yes, I would also prefer not to diverge between the x86 and arm versions
of xen_has_restricted_virtio_memory_access. But what case are we trying
to catch with (xen_pv_domain() || xen_hvm_domain()) ? Even on x86, it is
not going to leave much out. Is it really meant only to exclude pvh
domains?

I have the feeling that we could turn this check into:

static inline int xen_has_restricted_virtio_memory_access(void)
{
    return IS_ENABLED(CONFIG_XEN_VIRTIO) && xen_domain();
}

even on x86, but one of the xen/x86 maintainers should confirm.