linux-kernel - Re: [PATCH stable 5.9] KVM: arm64: ARM_SMCCC_ARCH_WORKAROUND_1 doesn't return SMCCC_RET_NOT

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Date:   Tue, 3 Nov 2020 21:21:57 +0100
From:   Greg KH <greg@...ah.com>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     stable@...r.kernel.org, linux-kernel@...r.kernel.org,
        Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        Andre Przywara <andre.przywara@....com>,
        Steven Price <steven.price@....com>
Subject: Re: [PATCH stable 5.9] KVM: arm64: ARM_SMCCC_ARCH_WORKAROUND_1
 doesn't return SMCCC_RET_NOT_REQUIRED

On Tue, Nov 03, 2020 at 12:15:26PM -0800, Stephen Boyd wrote:
> commit 1de111b51b829bcf01d2e57971f8fd07a665fa3f upstream.
> 
> According to the SMCCC spec[1](7.5.2 Discovery) the
> ARM_SMCCC_ARCH_WORKAROUND_1 function id only returns 0, 1, and
> SMCCC_RET_NOT_SUPPORTED.
> 
>  0 is "workaround required and safe to call this function"
>  1 is "workaround not required but safe to call this function"
>  SMCCC_RET_NOT_SUPPORTED is "might be vulnerable or might not be, who knows, I give up!"
> 
> SMCCC_RET_NOT_SUPPORTED might as well mean "workaround required, except
> calling this function may not work because it isn't implemented in some
> cases". Wonderful. We map this SMC call to
> 
>  0 is SPECTRE_MITIGATED
>  1 is SPECTRE_UNAFFECTED
>  SMCCC_RET_NOT_SUPPORTED is SPECTRE_VULNERABLE
> 
> For KVM hypercalls (hvc), we've implemented this function id to return
> SMCCC_RET_NOT_SUPPORTED, 0, and SMCCC_RET_NOT_REQUIRED. One of those
> isn't supposed to be there. Per the code we call
> arm64_get_spectre_v2_state() to figure out what to return for this
> feature discovery call.
> 
>  0 is SPECTRE_MITIGATED
>  SMCCC_RET_NOT_REQUIRED is SPECTRE_UNAFFECTED
>  SMCCC_RET_NOT_SUPPORTED is SPECTRE_VULNERABLE
> 
> Let's clean this up so that KVM tells the guest this mapping:
> 
>  0 is SPECTRE_MITIGATED
>  1 is SPECTRE_UNAFFECTED
>  SMCCC_RET_NOT_SUPPORTED is SPECTRE_VULNERABLE
> 
> Note: SMCCC_RET_NOT_AFFECTED is 1 but isn't part of the SMCCC spec
> 
> Fixes: c118bbb52743 ("arm64: KVM: Propagate full Spectre v2 workaround state to KVM guests")
> Signed-off-by: Stephen Boyd <swboyd@...omium.org>
> Acked-by: Marc Zyngier <maz@...nel.org>
> Acked-by: Will Deacon <will@...nel.org>
> Cc: Andre Przywara <andre.przywara@....com>
> Cc: Steven Price <steven.price@....com>
> Cc: Marc Zyngier <maz@...nel.org>
> Cc: stable@...r.kernel.org
> Link: https://developer.arm.com/documentation/den0028/latest [1]
> Link: https://lore.kernel.org/r/20201023154751.1973872-1-swboyd@chromium.org
> Signed-off-by: Will Deacon <will@...nel.org>
> Signed-off-by: Stephen Boyd <swboyd@...omium.org>
> ---
>  arch/arm64/kvm/hypercalls.c | 2 +-
>  include/linux/arm-smccc.h   | 2 ++
>  2 files changed, 3 insertions(+), 1 deletion(-)

Thanks for both of these, now queued up.

greg k-h