On 2013-03-07 12:06, Gleb Natapov wrote:
> On Thu, Mar 07, 2013 at 11:37:43AM +0100, Jan Kiszka wrote:
>> On 2013-03-07 09:57, Gleb Natapov wrote:
>>> On Thu, Mar 07, 2013 at 09:53:49AM +0100, Jan Kiszka wrote:
>>>> On 2013-03-07 09:43, Gleb Natapov wrote:
>>>>> On Thu, Mar 07, 2013 at 09:12:19AM +0100, Jan Kiszka wrote:
>>>>>> On 2013-03-07 08:51, Gleb Natapov wrote:
>>>>>>> On Mon, Mar 04, 2013 at 08:40:29PM +0100, Jan Kiszka wrote:
>>>>>>>> The logic for calculating the value with which we call kvm_set_cr0/4
>>>>>>>> was
>>>>>>>> broken (will definitely be visible with nested unrestricted guest mode
>>>>>>>> support). Also, we performed the check regarding CR0_ALWAYSON too early
>>>>>>>> when in guest mode.
>>>>>>>>
>>>>>>>> What really needs to be done on both CR0 and CR4 is to mask out
>>>>>>>> L1-owned
>>>>>>>> bits and merge them in from GUEST_CR0/4. In contrast, arch.cr0/4 and
>>>>>>>> arch.cr0/4_guest_owned_bits contain the mangled L0+L1 state and, thus,
>>>>>>>> are not suited as input.
>>>>>>>>
>>>>>>>> For both CRs, we can then apply the check against VMXON_CRx_ALWAYSON
>>>>>>>> and
>>>>>>>> refuse the update if it fails. To be fully consistent, we implement
>>>>>>>> this
>>>>>>>> check now also for CR4.
>>>>>>>>
>>>>>>>> Finally, we have to set the shadow to the value L2 wanted to write
>>>>>>>> originally.
>>>>>>>>
>>>>>>>> Signed-off-by: Jan Kiszka <jan.kis...@siemens.com>
>>>>>>>> ---
>>>>>>>>
>>>>>>>> Changes in v2:
>>>>>>>> - keep the non-misleading part of the comment in handle_set_cr0
>>>>>>>>
>>>>>>>> arch/x86/kvm/vmx.c | 46
>>>>>>>> +++++++++++++++++++++++++++++++---------------
>>>>>>>> 1 files changed, 31 insertions(+), 15 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>>>>>>> index 7cc566b..832b7b4 100644
>>>>>>>> --- a/arch/x86/kvm/vmx.c
>>>>>>>> +++ b/arch/x86/kvm/vmx.c
>>>>>>>> @@ -4605,37 +4605,53 @@ vmx_patch_hypercall(struct kvm_vcpu *vcpu,
>>>>>>>> unsigned char *hypercall)
>>>>>>>> /* called to set cr0 as appropriate for a mov-to-cr0 exit. */
>>>>>>>> static int handle_set_cr0(struct kvm_vcpu *vcpu, unsigned long val)
>>>>>>>> {
>>>>>>>> - if (to_vmx(vcpu)->nested.vmxon &&
>>>>>>>> - ((val & VMXON_CR0_ALWAYSON) != VMXON_CR0_ALWAYSON))
>>>>>>>> - return 1;
>>>>>>>> -
>>>>>>>> if (is_guest_mode(vcpu)) {
>>>>>>>> + struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
>>>>>>>> + unsigned long orig_val = val;
>>>>>>>> +
>>>>>>>> /*
>>>>>>>> * We get here when L2 changed cr0 in a way that did
>>>>>>>> not change
>>>>>>>> * any of L1's shadowed bits (see
>>>>>>>> nested_vmx_exit_handled_cr),
>>>>>>>> - * but did change L0 shadowed bits. This can currently
>>>>>>>> happen
>>>>>>>> - * with the TS bit: L0 may want to leave TS on (for
>>>>>>>> lazy fpu
>>>>>>>> - * loading) while pretending to allow the guest to
>>>>>>>> change it.
>>>>>>>> + * but did change L0 shadowed bits.
>>>>>>>> */
>>>>>>>> - if (kvm_set_cr0(vcpu, (val &
>>>>>>>> vcpu->arch.cr0_guest_owned_bits) |
>>>>>>>> - (vcpu->arch.cr0 &
>>>>>>>> ~vcpu->arch.cr0_guest_owned_bits)))
>>>>>>>> + val = (val & ~vmcs12->cr0_guest_host_mask) |
>>>>>>>> + (vmcs_read64(GUEST_CR0) &
>>>>>>>> vmcs12->cr0_guest_host_mask);
>>>>>>> I think using GUEST_CR0 here is incorrect. It contains combination of
>>>>>>> bits
>>>>>>> set by L2, L1 and L0 and here we need to get only L2/L1 mix which is in
>>>>>>> vcpu->arch.cr0 (almost, but good enough for this case). Why
>>>>>>> vcpu->arch.cr0
>>>>>>> contains right L2/L1 mix?
>>>>>>
>>>>>> L0/L1. E.g., kvm_set_cr0 unconditionally injects X86_CR0_ET and masks
>>>>>> out reserved bits. But you are right, GUEST_CR0 isn't much better. And
>>>>>> maybe that mangling in kvm_set_cr0 is a corner case we can ignore.
>>>>>>
>>>>> I think we can. ET is R/O and wired to 1, so it does not matter what
>>>>> guest writes there it should be treated as 1. About reserved bits spec
>>>>> says that software should write what it reads there and does not specify
>>>>> what happens if software does not follow this.
>>>>>
>>>>>>> Because it was set to vmcs12->guest_cr0 during
>>>>>>> L2 #vmentry. While L2 is running three things may happen to CR0:
>>>>>>>
>>>>>>> 1. L2 writes to a bit that is not shadowed neither by L1 nor by L0. It
>>>>>>> will go strait to GUEST_CR0.
>>>>>>> 2. L2 writes to a bit shadowed by L1. L1 #vmexit will be emulated. On
>>>>>>> the
>>>>>>> next #vmetry vcpu->arch.cr0 will be set to whatever value L1
>>>>>>> calculated.
>>>>>>> 3. L2 writes to a bit shadowed by L0, but not L1. This is the case we
>>>>>>> are handling here. And if we will do it right vcpu->arch.cr0 will be
>>>>>>> up-to-date at the end.
>>>>>>>
>>>>>>> The only case when, while this code running, vcpu->arch.cr0 has not
>>>>>>> up-to-date value is if 1 happened, but since L2 guest overwriting cr0
>>>>>>> here anyway it does not matter what it previously set in GUEST_CR0. The
>>>>>>> correct bits are in a new cr0 value provided by val and accessible by
>>>>>>> (val & ~vmcs12->cr0_guest_host_mask).
>>>>>>
>>>>>> I need to think about it again. Maybe vmcs12->guest_cr0 is best, but
>>>>>> that's a shot from the hips now.
>>>>>>
>>>>> I do not think it is correct because case 3 does not update it. So if 3
>>>>> happens twice without L1 #vmexit between then vmcs12->guest_cr0 will be
>>>>> outdated.
>>>>
>>>> Again, the only thing that matters here is L1's, not L0's view on the
>>>> "real" CR0 value. So guest_cr0 is never outdated (/wrt
>>>> cr0_guest_host_mask) as it will be updated by L1 in step 2. Even if
>>>> arch.cr0 vs. guest_cr0 makes no difference in practice, the latter is
>>>> more consistent, so I will go for it unless you can convince me it is
>>>> wrong.
>>>>
>>> Hmm, yes you are right that wrt cr0_guest_host_mask guest_cr0 should be
>>> up-to-date. Please write a big comment about it.
>>
>> Will do.
>>
>>> And what about moving VMXON_CR0_ALWAYSON check into vmx_set_cr0()?
>>
>> That doesn't make much sense for CR0 (due to the differences between
>> vmxon and guest mode - and lacking return code of set_cr4). But I can
>> consolidate the CR4 checks.
>>
> Isn't vmxon check is implicit in a guest mode. i.e if is_guest_mode() is
> trues then vmxon is on? Return code can be added.
Ah, sorry, you are not seeing what I'm looking at: The test will change
for L2 context once unrestricted guest mode is added. At that point, it
makes more sense to split it into one version that checks against
VMXON_CR0_ALWAYSON while in vmxon, targeting L1, and another that does
more complex evaluation for L2, depending on nested_cpu_has2(vmcs12,
SECONDARY_EXEC_UNRESTRICTED_GUEST).
Jan
--
Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
Corporate Competence Center Embedded Linux
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