[16/34] KVM: x86: "Reset" vcpu->run->exit_reason early in KVM_RUN

Commit Message

Paolo Bonzini Nov. 5, 2023, 4:30 p.m. UTC

From: Sean Christopherson <[e-mail]>

Initialize run->exit_reason to KVM_EXIT_UNKNOWN early in KVM_RUN to reduce
the probability of exiting to userspace with a stale run->exit_reason that
*appears* to be valid.

To support fd-based guest memory (guest memory without a corresponding
userspace virtual address), KVM will exit to userspace for various memory
related errors, which userspace *may* be able to resolve, instead of using
e.g. BUS_MCEERR_AR.  And in the more distant future, KVM will also likely
utilize the same functionality to let userspace "intercept" and handle
memory faults when the userspace mapping is missing, i.e. when fast gup()
fails.

Because many of KVM's internal APIs related to guest memory use '0' to
indicate "success, continue on" and not "exit to userspace", reporting
memory faults/errors to userspace will set run->exit_reason and
corresponding fields in the run structure fields in conjunction with a
a non-zero, negative return code, e.g. -EFAULT or -EHWPOISON.  And because
KVM already returns  -EFAULT in many paths, there's a relatively high
probability that KVM could return -EFAULT without setting run->exit_reason,
in which case reporting KVM_EXIT_UNKNOWN is much better than reporting
whatever exit reason happened to be in the run structure.

Note, KVM must wait until after run->immediate_exit is serviced to
sanitize run->exit_reason as KVM's ABI is that run->exit_reason is
preserved across KVM_RUN when run->immediate_exit is true.

Link: https://lore.kernel.org/all/[e-mail]
Link: https://lore.kernel.org/all/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>
Reviewed-by: Paolo Bonzini <[e-mail]>
Reviewed-by: Fuad Tabba <[e-mail]>
Tested-by: Fuad Tabba <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>
---
 arch/x86/kvm/x86.c | 1 +
 1 file changed, 1 insertion(+)

Comments

Xiaoyao Li Nov. 10, 2023, 8:49 a.m. UTC | #1

On 11/6/2023 12:30 AM, Paolo Bonzini wrote:
> From: Sean Christopherson <[e-mail]>
> 
> Initialize run->exit_reason to KVM_EXIT_UNKNOWN early in KVM_RUN to reduce
> the probability of exiting to userspace with a stale run->exit_reason that
> *appears* to be valid.
> 
> To support fd-based guest memory (guest memory without a corresponding
> userspace virtual address), KVM will exit to userspace for various memory
> related errors, which userspace *may* be able to resolve, instead of using
> e.g. BUS_MCEERR_AR.  And in the more distant future, KVM will also likely
> utilize the same functionality to let userspace "intercept" and handle
> memory faults when the userspace mapping is missing, i.e. when fast gup()
> fails.
> 
> Because many of KVM's internal APIs related to guest memory use '0' to
> indicate "success, continue on" and not "exit to userspace", reporting
> memory faults/errors to userspace will set run->exit_reason and
> corresponding fields in the run structure fields in conjunction with a
> a non-zero, negative return code, e.g. -EFAULT or -EHWPOISON.  And because
> KVM already returns  -EFAULT in many paths, there's a relatively high
> probability that KVM could return -EFAULT without setting run->exit_reason,
> in which case reporting KVM_EXIT_UNKNOWN is much better than reporting
> whatever exit reason happened to be in the run structure.
> 
> Note, KVM must wait until after run->immediate_exit is serviced to
> sanitize run->exit_reason as KVM's ABI is that run->exit_reason is
> preserved across KVM_RUN when run->immediate_exit is true.
> 
> Link: https://lore.kernel.org/all/[e-mail]
> Link: https://lore.kernel.org/all/[e-mail]
> Signed-off-by: Sean Christopherson <[e-mail]>
> Reviewed-by: Paolo Bonzini <[e-mail]>
> Reviewed-by: Fuad Tabba <[e-mail]>
> Tested-by: Fuad Tabba <[e-mail]>
> Message-Id: <[e-mail]>
> Signed-off-by: Paolo Bonzini <[e-mail]>

Reviewed-by: Xiaoyao Li <[e-mail]>

> ---
>   arch/x86/kvm/x86.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8f9d8939b63b..f661acb01c58 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11082,6 +11082,7 @@ static int vcpu_run(struct kvm_vcpu *vcpu)
>   {
>   	int r;
>   
> +	vcpu->run->exit_reason = KVM_EXIT_UNKNOWN;
>   	vcpu->arch.l1tf_flush_l1d = true;
>   
>   	for (;;) {

Patch

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 8f9d8939b63b..f661acb01c58 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11082,6 +11082,7 @@  static int vcpu_run(struct kvm_vcpu *vcpu)
 {
 	int r;
 
+	vcpu->run->exit_reason = KVM_EXIT_UNKNOWN;
 	vcpu->arch.l1tf_flush_l1d = true;
 
 	for (;;) {