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    SubjectRe: [PATCH v1 3/3] x86/bugs: Switch the selection of mitigation from CPU vendor to CPU features
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    On 6/1/2018 9:59 AM, Konrad Rzeszutek Wilk wrote:
    > Both AMD and Intel can have SPEC CTRL MSR for SSBD.
    >
    > However AMD also has two more other ways of doing it - which
    > are !SPEC_CTRL MSR ways.
    >
    > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
    >
    > ---
    > Cc: Thomas Gleixner <tglx@linutronix.de>
    > Cc: Ingo Molnar <mingo@redhat.com>
    > Cc: "H. Peter Anvin" <hpa@zytor.com>
    > Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
    > Cc: Borislav Petkov <bp@suse.de>
    > Cc: David Woodhouse <dwmw@amazon.co.uk>
    > Cc: Kees Cook <keescook@chromium.org>
    > Cc: KarimAllah Ahmed <karahmed@amazon.de>
    > ---
    > arch/x86/kernel/cpu/bugs.c | 11 +++--------
    > 1 file changed, 3 insertions(+), 8 deletions(-)
    >
    > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
    > index 6bea81855cdd..cd0fda1fff6d 100644
    > --- a/arch/x86/kernel/cpu/bugs.c
    > +++ b/arch/x86/kernel/cpu/bugs.c
    > @@ -532,17 +532,12 @@ static enum ssb_mitigation __init __ssb_select_mitigation(void)
    > * Intel uses the SPEC CTRL MSR Bit(2) for this, while AMD may
    > * use a completely different MSR and bit dependent on family.
    > */
    > - switch (boot_cpu_data.x86_vendor) {
    > - case X86_VENDOR_INTEL:
    > - case X86_VENDOR_AMD:
    > - if (!static_cpu_has(X86_FEATURE_MSR_SPEC_CTRL)) {
    > - x86_amd_ssb_disable();
    > - break;
    > - }
    > + if (!static_cpu_has(X86_FEATURE_MSR_SPEC_CTRL))
    > + x86_amd_ssb_disable();
    > + else {

    As I think about this more, I don't think we can do this for AMD. The
    X86_FEATURE_SSBD could be true because of the LS_CFG support and not the
    AMD_SSBD CPUID bit. But if the IBRS CPUID bit was set, we would now try
    to use the SPEC_CTRL register for SSBD, which is not valid.

    I think you will have to keep the case statements and explicitly check for
    X86_FEATURE_AMD_SSBD before using SPEC_CTRL.

    Thanks,
    Tom

    > x86_spec_ctrl_base |= SPEC_CTRL_SSBD;
    > x86_spec_ctrl_mask |= SPEC_CTRL_SSBD;
    > wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
    > - break;
    > }
    > }
    >
    >

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