linux-kernel - Re: [PATCH v5 11/14] x86/sev: Prevent RDTSC/RDTSCP interception for Secure TSC enabled guests

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Message-ID: <9acf1f56-f8d8-40e3-8fec-fa982db1a7a1@amd.com>
Date:   Thu, 2 Nov 2023 11:17:55 +0530
From:   "Nikunj A. Dadhania" <nikunj@....com>
To:     Tom Lendacky <thomas.lendacky@....com>,
        linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org
Cc:     bp@...en8.de, mingo@...hat.com, tglx@...utronix.de,
        dave.hansen@...ux.intel.com, dionnaglaze@...gle.com,
        pgonda@...gle.com, seanjc@...gle.com, pbonzini@...hat.com
Subject: Re: [PATCH v5 11/14] x86/sev: Prevent RDTSC/RDTSCP interception for
 Secure TSC enabled guests

On 10/31/2023 2:02 AM, Tom Lendacky wrote:
> On 10/30/23 01:36, Nikunj A Dadhania wrote:
>> The hypervisor should not be intercepting RDTSC/RDTSCP when Secure TSC
>> is enabled. A #VC exception will be generated if the RDTSC/RDTSCP
>> instructions are being intercepted. If this should occur and Secure
>> TSC is enabled, terminate guest execution.
>>
>> Signed-off-by: Nikunj A Dadhania <nikunj@....com>
>> ---
>>   arch/x86/kernel/sev-shared.c | 7 +++++++
>>   1 file changed, 7 insertions(+)
>>
>> diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
>> index ccb0915e84e1..833b0ae38f0b 100644
>> --- a/arch/x86/kernel/sev-shared.c
>> +++ b/arch/x86/kernel/sev-shared.c
>> @@ -991,6 +991,13 @@ static enum es_result vc_handle_rdtsc(struct ghcb *ghcb,
>>       bool rdtscp = (exit_code == SVM_EXIT_RDTSCP);
>>       enum es_result ret;
>>   +    /*
>> +     * RDTSC and RDTSCP should not be intercepted when Secure TSC is
>> +     * enabled. Terminate the SNP guest when the interception is enabled.
>> +     */
>> +    if (sev_status & MSR_AMD64_SNP_SECURE_TSC)
> 
> If you have to use sev_status, then please document why cc_platform_has() can't be used in the comment above.

Right, for sev-shared.c, cc_platform_has() is not available when compiling boot/compressed.

Regards
Nikunj