On Fri, Apr 17, 2020 at 04:50:02PM +0100, Andrew Cooper wrote:
> This is the start of some performance and security-hardening improvements,
> based on the fact that 32bit PV guests are few and far between these days.
>
> Ring1 is full or architectural corner cases, such as counting as supervisor
^ of
> from a paging point of view. This accounts for a substantial performance hit
> on processors from the last 8 years (adjusting SMEP/SMAP on every privilege
> transition), and the gap is only going to get bigger with new hardware
> features.
>
> Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com>
> ---
> CC: Jan Beulich <jbeul...@suse.com>
> CC: Wei Liu <w...@xen.org>
> CC: Roger Pau Monné <roger....@citrix.com>
>
> There is a series I can't quite post yet which wants to conditionally turn
> opt_pv32 off, which is why I've put it straight in in an int8_t form rather
s/in in/in/
> than a straight boolean form.
> ---
> docs/misc/xen-command-line.pandoc | 12 +++++++++++-
> xen/arch/x86/Kconfig | 16 ++++++++++++++++
> xen/arch/x86/pv/domain.c | 35 +++++++++++++++++++++++++++++++++++
> xen/arch/x86/setup.c | 9 +++++++--
> xen/include/asm-x86/pv/domain.h | 6 ++++++
> 5 files changed, 75 insertions(+), 3 deletions(-)
>
> diff --git a/docs/misc/xen-command-line.pandoc
> b/docs/misc/xen-command-line.pandoc
> index acd0b3d994..ee12b0f53f 100644
> --- a/docs/misc/xen-command-line.pandoc
> +++ b/docs/misc/xen-command-line.pandoc
> @@ -1694,7 +1694,17 @@ The following resources are available:
> CDP, one COS will corespond two CBMs other than one with CAT, due to the
> sum of CBMs is fixed, that means actual `cos_max` in use will
> automatically
> reduce to half when CDP is enabled.
> -
> +
> +### pv
> + = List of [ 32=<bool> ]
> +
> + Applicability: x86
> +
> +Controls for aspects of PV guest support.
> +
> +* The `32` boolean controls whether 32bit PV guests can be created. It
> + defaults to `true`, and is ignored when `CONFIG_PV32` is compiled out.
> +
> ### pv-linear-pt (x86)
> > `= <boolean>`
>
> diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
> index 8149362bde..4c52197de3 100644
> --- a/xen/arch/x86/Kconfig
> +++ b/xen/arch/x86/Kconfig
> @@ -49,6 +49,22 @@ config PV
>
> If unsure, say Y.
>
> +config PV32
> + bool "Support for 32bit PV guests"
> + depends on PV
> + default y
> + ---help---
> + The 32bit PV ABI uses Ring1, an area of the x86 architecture which
> + was deprecated and mostly removed in the AMD64 spec. As a result,
> + it occasionally conflicts with newer x86 hardware features, causing
> + overheads for Xen to maintain backwards compatibility.
> +
> + People may wish to disable 32bit PV guests for attack surface
> + reduction, or performance reasons. Backwards compatibility can be
> + provided via the PV Shim mechanism.
> +
> + If unsure, say Y.
> +
> config PV_LINEAR_PT
> bool "Support for PV linear pagetables"
> depends on PV
> diff --git a/xen/arch/x86/pv/domain.c b/xen/arch/x86/pv/domain.c
> index 70fae43965..47a0db082f 100644
> --- a/xen/arch/x86/pv/domain.c
> +++ b/xen/arch/x86/pv/domain.c
> @@ -16,6 +16,39 @@
> #include <asm/pv/domain.h>
> #include <asm/shadow.h>
>
> +#ifdef CONFIG_PV32
> +int8_t __read_mostly opt_pv32 = -1;
> +#endif
> +
> +static int parse_pv(const char *s)
__init
With that:
Reviewed-by: Roger Pau Monné <roger....@citrix.com>
Thanks, Roger.
