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Wed, 20 May 2026 07:25:35 -0700 (PDT) Message-ID: Date: Wed, 20 May 2026 16:25:34 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/3] x86/shadow: VRAM last_dirty tagging To: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= Cc: "xen-devel@lists.xenproject.org" , Andrew Cooper References: <5149ab32-7d03-4ae5-9af0-e62dd5836329@suse.com> <8559db88-5f1d-4ced-980c-e71c4e229c7c@suse.com> Content-Language: en-US From: Jan Beulich Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-purgate-ID: tlsNG-d25034/1779287135-DBF7ACF5-534AB3F1/0/0 X-purgate-type: clean X-purgate-size: 2997 On 20.05.2026 15:54, Roger Pau Monné wrote: > On Mon, May 04, 2026 at 10:39:53AM +0200, Jan Beulich wrote: >> On 27.04.2026 11:28, Roger Pau Monné wrote: >>> On Tue, Feb 03, 2026 at 05:49:55PM +0100, Jan Beulich wrote: >>>> --- a/xen/arch/x86/mm/shadow/hvm.c >>>> +++ b/xen/arch/x86/mm/shadow/hvm.c >>>> @@ -1087,18 +1087,18 @@ int shadow_track_dirty_vram(struct domai >>>> if ( (dirty_vram->dirty_bitmap = xzalloc_array(uint8_t, dirty_size)) == NULL ) >>>> goto out_sl1ma; >>>> >>>> - dirty_vram->last_dirty = NOW(); >>>> + dirty_vram->last_dirty = -1; >>>> >>>> /* Tell the caller that this time we could not track dirty bits. */ >>>> rc = -ENODATA; >>>> } >>>> - else if ( dirty_vram->last_dirty == -1 ) >>>> - /* still completely clean, just copy our empty bitmap */ >>>> - memcpy(dirty_bitmap, dirty_vram->dirty_bitmap, dirty_size); >>>> - else >>>> + /* Nothing to do when the bitmap is still completely clean. */ >>>> + else if ( dirty_vram->last_dirty != -1 ) >>>> { >>>> mfn_t map_mfn = INVALID_MFN; >>>> void *map_sl1p = NULL; >>>> + bool any_dirty = false; >>>> + s_time_t now; >>>> >>>> /* Iterate over VRAM to track dirty bits. */ >>>> for ( i = 0; i < nr_frames; i++ ) >>>> @@ -1174,16 +1174,20 @@ int shadow_track_dirty_vram(struct domai >>>> if ( dirty ) >>>> { >>>> dirty_vram->dirty_bitmap[i / 8] |= 1 << (i % 8); >>>> - dirty_vram->last_dirty = NOW(); >>>> + any_dirty = true; >>>> } >>>> } >>>> >>>> + now = NOW(); >>>> + if ( any_dirty ) >>>> + dirty_vram->last_dirty = now; >>> >>> I'm a bit confused with the setting of ->last_dirty here ... >>> >>>> + >>>> if ( map_sl1p ) >>>> unmap_domain_page(map_sl1p); >>>> >>>> memcpy(dirty_bitmap, dirty_vram->dirty_bitmap, dirty_size); >>>> memset(dirty_vram->dirty_bitmap, 0, dirty_size); >>> >>> ... as here the bitmap is zeroed, and hence ->last_dirty should be set >>> to -1? >> >> That's not how I understand the field is used. Aiui it identifies "was >> clean for more than 2 seconds". That's not the case here. Hence the >> setting to -1 only conditionally a few lines down from here. > > Hm, OK, it seems like a very complicated way to signal this. Won't it > be easier to unconditionally store the last write time in > ->last_dirty, and let the consumer decide whether it's been more than > 2s or not? > > Maybe you could write a comment next to the field in the struct > declaration? Maybe (in a separate patch), but then how I understand the field is used may still not be quite correct. I.e. by adding a comment I may further confuse things. > Either way: > > Acked-by: Roger Pau Monné Thanks. Jan