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SubjectRe: [PATCH fsgsbase v2 4/4] x86/fsgsbase: Fix Xen PV support
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On 26.06.20 19:24, Andy Lutomirski wrote:
> On Xen PV, SWAPGS doesn't work. Teach __rdfsbase_inactive() and
> __wrgsbase_inactive() to use rdmsrl()/wrmsrl() on Xen PV. The Xen
> pvop code will understand this and issue the correct hypercalls.
>
> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: Stefano Stabellini <sstabellini@kernel.org>
> Cc: xen-devel@lists.xenproject.org
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
> arch/x86/kernel/process_64.c | 20 ++++++++++++++------
> 1 file changed, 14 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index cb8e37d3acaa..457d02aa10d8 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -163,9 +163,13 @@ static noinstr unsigned long __rdgsbase_inactive(void)
>
> lockdep_assert_irqs_disabled();
>
> - native_swapgs();
> - gsbase = rdgsbase();
> - native_swapgs();
> + if (!static_cpu_has(X86_FEATURE_XENPV)) {
> + native_swapgs();
> + gsbase = rdgsbase();
> + native_swapgs();
> + } else {
> + rdmsrl(MSR_KERNEL_GS_BASE, gsbase);
> + }
>
> return gsbase;
> }
> @@ -182,9 +186,13 @@ static noinstr void __wrgsbase_inactive(unsigned long gsbase)
> {
> lockdep_assert_irqs_disabled();
>
> - native_swapgs();
> - wrgsbase(gsbase);
> - native_swapgs();
> + if (!static_cpu_has(X86_FEATURE_XENPV)) {
> + native_swapgs();
> + wrgsbase(gsbase);
> + native_swapgs();
> + } else {
> + wrmsrl(MSR_KERNEL_GS_BASE, gsbase);
> + }
> }
>
> /*
>

Another possibility would be to just do (I'm fine either way):

diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index acc49fa6a097..b78dd373adbf 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -318,6 +318,8 @@ static void __init xen_init_capabilities(void)
setup_clear_cpu_cap(X86_FEATURE_XSAVE);
setup_clear_cpu_cap(X86_FEATURE_OSXSAVE);
}
+
+ setup_clear_cpu_cap(X86_FEATURE_FSGSBASE);
}

static void xen_set_debugreg(int reg, unsigned long val)

Juergen

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