linux-kernel - Re: [PATCH v2 2/3] xen/pv: refactor msr access functions to support safe and unsafe accesses

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Message-ID: <ee01b8d6-ee0c-b17d-1969-4aed41fa8666@suse.com>
Date:   Tue, 4 Oct 2022 17:33:58 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v2 2/3] xen/pv: refactor msr access functions to support
 safe and unsafe accesses

On 04.10.22 13:03, Jan Beulich wrote:
> On 04.10.2022 10:43, Juergen Gross wrote:
>> Refactor and rename xen_read_msr_safe() and xen_write_msr_safe() to
>> support both cases of MSR accesses, safe ones and potentially GP-fault
>> generating ones.
>>
>> This will prepare to no longer swallow GPs silently in xen_read_msr()
>> and xen_write_msr().
>>
>> Signed-off-by: Juergen Gross <jgross@...e.com>
> 
> Reviewed-by: Jan Beulich <jbeulich@...e.com>
> 
> Albeit ...
> 
>> @@ -933,23 +937,39 @@ static u64 xen_read_msr_safe(unsigned int msr, int *err)
>>   	return val;
>>   }
>>   
>> -static int xen_write_msr_safe(unsigned int msr, unsigned low, unsigned high)
>> +static void set_seg(unsigned int which, unsigned int low, unsigned int high,
>> +		    int *err)
>>   {
>> -	int ret;
>> -	unsigned int which;
>> -	u64 base;
>> +	u64 base = ((u64)high << 32) | low;
>> +
>> +	if (HYPERVISOR_set_segment_base(which, base) == 0)
>> +		return;
>>   
>> -	ret = 0;
>> +	if (err)
>> +		*err = -EIO;
> 
> ... I don't see a good reason to override the error code handed to us
> by the hypervisor here; I do realize though that this ...
> 
>> +	else
>> +		WARN(1, "Xen set_segment_base(%u, %llx) failed\n", which, base);
>> +}
>>   
>> +/*
>> + * Support write_msr_safe() and write_msr() semantics.
>> + * With err == NULL write_msr() semantics are selected.
>> + * Supplying an err pointer requires err to be pre-initialized with 0.
>> + */
>> +static void xen_do_write_msr(unsigned int msr, unsigned int low,
>> +			     unsigned int high, int *err)
>> +{
>>   	switch (msr) {
>> -	case MSR_FS_BASE:		which = SEGBASE_FS; goto set;
>> -	case MSR_KERNEL_GS_BASE:	which = SEGBASE_GS_USER; goto set;
>> -	case MSR_GS_BASE:		which = SEGBASE_GS_KERNEL; goto set;
>> -
>> -	set:
>> -		base = ((u64)high << 32) | low;
>> -		if (HYPERVISOR_set_segment_base(which, base) != 0)
>> -			ret = -EIO;
> 
> ... was this way before.

And on bare metal write_msr_safe() will return -EIO, too.


Juergen

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