Re: [PATCH] target/i386: Expose IBPB-BRTYPE and SBPB CPUID bits to the guest

[Fabiano Rosas <farosas@suse.de>]

Fabiano Rosas <farosas@suse.de> writes:

> According to AMD's Speculative Return Stack Overflow whitepaper (link
> below), the hypervisor should synthesize the value of IBPB_BRTYPE and
> SBPB CPUID bits to the guest.
>
> Support for this is already present in the kernel with commit
> e47d86083c66 ("KVM: x86: Add SBPB support") and commit 6f0f23ef76be
> ("KVM: x86: Add IBPB_BRTYPE support").
>
> Add support in QEMU to expose the bits to the guest OS.
>
> host:
>   # cat /sys/devices/system/cpu/vulnerabilities/spec_rstack_overflow
>   Mitigation: Safe RET
>
> before (guest):
>   $ cpuid -l 0x80000021 -1 -r
>   0x80000021 0x00: eax=0x00000045 ebx=0x00000000 ecx=0x00000000 edx=0x00000000
>                             ^
>   $ cat /sys/devices/system/cpu/vulnerabilities/spec_rstack_overflow
>   Vulnerable: Safe RET, no microcode
>
> after (guest):
>   $ cpuid -l 0x80000021 -1 -r
>   0x80000021 0x00: eax=0x18000045 ebx=0x00000000 ecx=0x00000000 edx=0x00000000
>                             ^
>   $ cat /sys/devices/system/cpu/vulnerabilities/spec_rstack_overflow
>   Mitigation: Safe RET
>
> Reported-by: Fabian Vogt <fvogt@suse.de>
> Link: https://www.amd.com/content/dam/amd/en/documents/corporate/cr/speculative-return-stack-overflow-whitepaper.pdf
> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> ---
> More info on this thread:
> https://lore.kernel.org/r/68f8b8b1ca1bf58b059f52afbd1c9c51108a074a.camel@suse.com
> ---
>  target/i386/cpu.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 85ef7452c0..d33401c922 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -1221,8 +1221,8 @@ FeatureWordInfo feature_word_info[FEATURE_WORDS] = {
>              NULL, NULL, NULL, NULL,
>              NULL, NULL, NULL, NULL,
>              NULL, NULL, NULL, NULL,
> -            NULL, NULL, NULL, NULL,
> -            NULL, NULL, NULL, NULL,
> +            NULL, NULL, NULL, "sbpb",
> +            "ibpb-brtype", NULL, NULL, NULL,
>          },
>          .cpuid = { .eax = 0x80000021, .reg = R_EAX, },
>          .tcg_features = 0,

Ping, any thoughts on this one?