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From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: "Roger Pau Monné" <roger.pau@citrix.com>, "Wei Liu" <wl@xen.org>,
	Xen-devel <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH 5/5] x86/cpuid: Fix handling of xsave dynamic leaves
Date: Tue, 4 May 2021 15:17:10 +0100	[thread overview]
Message-ID: <1279476a-f99d-59a4-7fed-1aee37dbe204@citrix.com> (raw)
In-Reply-To: <5e6511ca-83bd-8a43-202e-949b4d19b1ab@suse.com>

On 04/05/2021 13:56, Jan Beulich wrote:
> On 03.05.2021 17:39, Andrew Cooper wrote:
>> If max leaf is greater than 0xd but xsave not available to the guest, then the
>> current XSAVE size should not be filled in.  This is a latent bug for now as
>> the guest max leaf is 0xd, but will become problematic in the future.
>>
>> The comment concerning XSS state is wrong.  VT-x doesn't manage host/guest
>> state automatically, but there is provision for "host only" bits to be set, so
>> the implications are still accurate.
>>
>> Introduce {xstate,hw}_compressed_size() helpers to mirror the uncompressed
>> ones.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> Reviewed-by: Jan Beulich <jbeulich@suse.com>
> albeit with a remark:
>
>> +unsigned int xstate_compressed_size(uint64_t xstates)
>> +{
>> +    unsigned int i, size = XSTATE_AREA_MIN_SIZE;
>> +
>> +    xstates &= ~XSTATE_FP_SSE;
>> +    for_each_set_bit ( i, &xstates, 63 )
>> +    {
>> +        if ( test_bit(i, &xstate_align) )
>> +            size = ROUNDUP(size, 64);
>> +
>> +        size += xstate_sizes[i];
>> +    }
>> +
>> +    /* In debug builds, cross-check our calculation with hardware. */
>> +    if ( IS_ENABLED(CONFIG_DEBUG) )
>> +    {
>> +        unsigned int hwsize;
>> +
>> +        xstates |= XSTATE_FP_SSE;
>> +        hwsize = hw_compressed_size(xstates);
>> +
>> +        if ( size != hwsize )
>> +            printk_once(XENLOG_ERR "%s(%#"PRIx64") size %#x != hwsize %#x\n",
>> +                        __func__, xstates, size, hwsize);
>> +        size = hwsize;
> To be honest, already on the earlier patch I was wondering whether
> it does any good to override size here: That'll lead to different
> behavior on debug vs release builds. If the log message is not
> paid attention to, we'd then end up with longer term breakage.

Well - our options are pass hardware size, or BUG(), because getting
this wrong will cause memory corruption.

The BUG() option is a total pain when developing new support - the first
version of this patch did use BUG(), but after hitting it 4 times in a
row (caused by issues with constants elsewhere), I decided against it.

If we had something which was a mix between WARN_ONCE() and a proper
printk() explaining what was going on, then I would have used that. 
Maybe it's time to introduce one...

~Andrew



  reply	other threads:[~2021-05-04 14:17 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-03 15:39 [PATCH 0/5] x86/xstate: Fixes to size calculations Andrew Cooper
2021-05-03 15:39 ` [PATCH 1/5] x86/xstate: Elide redundant writes in set_xcr0() Andrew Cooper
2021-05-04 11:51   ` Jan Beulich
2021-05-03 15:39 ` [PATCH 2/5] x86/xstate: Rename _xstate_ctxt_size() to hw_uncompressed_size() Andrew Cooper
2021-05-04 11:53   ` Jan Beulich
2021-05-03 15:39 ` [PATCH 3/5] x86/xstate: Rework xstate_ctxt_size() as xstate_uncompressed_size() Andrew Cooper
2021-05-03 18:17   ` Andrew Cooper
2021-05-04 12:08     ` Jan Beulich
2021-05-04 12:15       ` Andrew Cooper
2021-05-04 12:20   ` Jan Beulich
2021-05-04 12:22     ` Andrew Cooper
2021-05-04 12:45       ` Jan Beulich
2021-05-03 15:39 ` [PATCH 4/5] x86/cpuid: Simplify recalculate_xstate() Andrew Cooper
2021-05-04 12:43   ` Jan Beulich
2021-05-04 13:58     ` Andrew Cooper
2021-05-05  8:19       ` Jan Beulich
2021-05-05 14:53         ` Andrew Cooper
2021-05-05 15:14           ` Jan Beulich
2021-05-03 15:39 ` [PATCH 5/5] x86/cpuid: Fix handling of xsave dynamic leaves Andrew Cooper
2021-05-04 12:56   ` Jan Beulich
2021-05-04 14:17     ` Andrew Cooper [this message]
2021-05-05  8:33       ` Jan Beulich
2021-05-05 16:59         ` Andrew Cooper
2021-05-06  6:17           ` Jan Beulich

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