linux-kernel - Re: [PATCH v2 1/8] KVM: x86: Ignore sparse banks size for an "all CPUs", non-sparse IPI req

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Message-ID: <87fssgkzzd.fsf@vitty.brq.redhat.com>
Date:   Mon, 01 Nov 2021 10:05:26 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        Arnd Bergmann <arnd@...db.de>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-hyperv@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ajay Garg <ajaygargnsit@...il.com>
Subject: Re: [PATCH v2 1/8] KVM: x86: Ignore sparse banks size for an "all
 CPUs", non-sparse IPI req

Sean Christopherson <seanjc@...gle.com> writes:

> Do not bail early if there are no bits set in the sparse banks for a
> non-sparse, a.k.a. "all CPUs", IPI request.  Per the Hyper-V spec, it is
> legal to have a variable length of '0', e.g. VP_SET's BankContents in
> this case, if the request can be serviced without the extra info.
>
>   It is possible that for a given invocation of a hypercall that does
>   accept variable sized input headers that all the header input fits
>   entirely within the fixed size header. In such cases the variable sized
>   input header is zero-sized and the corresponding bits in the hypercall
>   input should be set to zero.
>
> Bailing early results in KVM failing to send IPIs to all CPUs as expected
> by the guest.
>
> Fixes: 214ff83d4473 ("KVM: x86: hyperv: implement PV IPI send hypercalls")
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/hyperv.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index 4f15c0165c05..814d1a1f2cb8 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -1922,11 +1922,13 @@ static u64 kvm_hv_send_ipi(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc, bool
>  
>  		all_cpus = send_ipi_ex.vp_set.format == HV_GENERIC_SET_ALL;
>  
> +		if (all_cpus)
> +			goto check_and_send_ipi;
> +
>  		if (!sparse_banks_len)
>  			goto ret_success;
>  
> -		if (!all_cpus &&
> -		    kvm_read_guest(kvm,
> +		if (kvm_read_guest(kvm,
>  				   hc->ingpa + offsetof(struct hv_send_ipi_ex,
>  							vp_set.bank_contents),
>  				   sparse_banks,
> @@ -1934,6 +1936,7 @@ static u64 kvm_hv_send_ipi(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc, bool
>  			return HV_STATUS_INVALID_HYPERCALL_INPUT;
>  	}
>  
> +check_and_send_ipi:
>  	if ((vector < HV_IPI_LOW_VECTOR) || (vector > HV_IPI_HIGH_VECTOR))
>  		return HV_STATUS_INVALID_HYPERCALL_INPUT;

Nice catch! It's possible that genuine Windows/Hyper-V guests never hit
the bug because they don't use 'ex' version to send IPIs to all CPUs. It
is also possible that Windows/Hyper-V guests with > 64 vCPUs are just
undertested.

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly