Re: [PATCH v6 4/5] x86/kvm: use __decrypted attribute in shared variables

[Paolo Bonzini]

On 07/09/2018 19:57, Brijesh Singh wrote:
> Commit: 368a540e0232 (x86/kvmclock: Remove memblock dependency)
> caused SEV guest regression. When SEV is active, we map the shared
> variables (wall_clock and hv_clock_boot) with C=0 to ensure that both
> the guest and the hypervisor are able to access the data. To map the
> variables we use kernel_physical_mapping_init() to split the large pages,
> but splitting large pages requires allocating a new PMD, which fails now
> that kvmclock initialization is called early during boot.
> 
> Recently we added a special .data..decrypted section to hold the shared
> variables. This section is mapped with C=0 early during boot. Use
> __decrypted attribute to put the wall_clock and hv_clock_boot in
> .data..decrypted section so that they are mapped with C=0.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Fixes: 368a540e0232 ("x86/kvmclock: Remove memblock dependency")
> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx
> Cc: "Radim KrÄmÃÅ" <rkrcmar@xxxxxxxxxx>
> ---
>  arch/x86/kernel/kvmclock.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index 1e67646..376fd3a 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -61,8 +61,8 @@ early_param("no-kvmclock-vsyscall", parse_no_kvmclock_vsyscall);
>  	(PAGE_SIZE / sizeof(struct pvclock_vsyscall_time_info))
>  
>  static struct pvclock_vsyscall_time_info
> -			hv_clock_boot[HVC_BOOT_ARRAY_SIZE] __aligned(PAGE_SIZE);
> -static struct pvclock_wall_clock wall_clock;
> +			hv_clock_boot[HVC_BOOT_ARRAY_SIZE] __decrypted __aligned(PAGE_SIZE);
> +static struct pvclock_wall_clock wall_clock __decrypted;
>  static DEFINE_PER_CPU(struct pvclock_vsyscall_time_info *, hv_clock_per_cpu);
>  
>  static inline struct pvclock_vcpu_time_info *this_cpu_pvti(void)
> 

Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

(Though perhaps __noencrypt or __unencrypted would be a bit more
accurate; likewise for the freeing function added in patch 5).

Paolo